During Proto-Germanic

Verner's law shifted Proto-Germanic /*h/ > /*g/ after an unstressed syllable. Afterwards, stress shifted to the first syllable in all words.^[3] In many Old Norse verbs, a lost /g/ reappears in the forms of some verbs, which makes their morphology abnormal, but remain regular because the forms containing /g/s are the same for each verb they appear in.

ex.: Proto-Germanic *slōhúm > *slōgúm (we struck) had an unstressed first syllable, but the corresponding singular, *slṓh (I struck), had only a stressed syllable. These became Old Norse slógum and sló.^[3]:1

Before Old Norse

Umlaut was originally an assimilation of root vowels to suffixes having the front phonemes /i, j/ (i-umlaut) and the back phonemes /u, w/ (u-umlaut) in Proto-Germanic. The suffixes were not on all forms of the same words, so when the suffixes underwent syncope during the transition into Old Norse, the remaining umlaut of the vowel indicated what the suffix had before. From then on speakers would alternate the vowel as an act of inflection.

Strong verbs

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Strong verbs, unlike weak verbs, are conjugated by ablaut, a process that replaces, rather than modifies, their root vowel. The English sing uses ablaut to conjugate to sang in the past tense and sung as the past participle. Like weak verbs, strong verbs use inflections and umlaut, but they rely on them much less to distinguish forms and conjugations. While the strongs' umlaut and inflectional patterns are largely the same from verb to verb, there are different sets and numbers of vowels involved in ablaut, and so their patterns are used to classify the strong conjugations.

If there are 2 vowels in the pattern (as in the 6th & some 7th conjugation patterns), the 2nd is used for all the past forms. If there are 3, the 2nd vowel is used for the indicative past singulars, & the 3rd for the other past tenses. The 1st vowel is used for the remaining forms: the infinitive, present forms, and imperative, and usually the past participle of 3-vowel words. However, some 3-vowel words have a 4th vowel appearing only in the participle.

The past participle of strong verbs follows the pronominal declension of hit,^{[cv 5]} though unlike the definite suffix the participle is inherited from Proto-Germanic.

The 3rd, 4th and 5th conjugations have an i, rather than an a, in the 1st person subjunctive past ending. Third conjugation words ending in n, g, or k have a u for their past participles' root vowel. The jas of the 3rd conjugation are due to breaking. The 4th and 5th conjugations are identical except in the past participle, where the 4th conjugation normally has o and the fifth conjugation e. Generally, 3rd conjugation stems have two consonants following the vowel; 4th conjugation stems have a single sonorant consonant (l, r, m or n) following the vowel; and 5th conjugation stems have a single consonant that is not a sonorant.

The 6th conjugation is cognate with English's take/took/taken conjugation. The 7th conjugation is a heterogenous category. Its ablaut patterns include a/á, e/é; au, jó; a, jó, jo; and ý, jó, ú.

Weak verbs

Weak verbs distinguish the tenses of the indicative and subjunctive primarily by adding a suffix with a dental (t, d, or ð). This is the primary mode of distinction of tenses, in contrast with the radical vowel changes characteristic of the strong verbs. Preceded by the dental, the subjunctive past tense endings take the form of their present tense endings, changing the inflectional vowel to i in the plural. The indicative forms take the subjunctive forms, changing the inflectional vowel to u in the plural. The dental is preceded by an a in some verbs, causing the past tenses to become trisyllabic.

There are three primary conjugations, corresponding approximately to the first three classes of weak verbs in Gothic. The Proto-Germanic and Gothic Class IV weak verb, with a *-n(ō)- suffix, has been incorporated into the second conjugation in Old Norse.

Present-preterite verbs

Present-preterite, or present-in-past, verbs form their present tenses using the ablaut patterns of strong verbs' past tenses. Their past tenses are formed like weak verbs'.

Suffixes and clitics

This section needs expansion with: The personal suffix. You can help by adding to it. (August 2011)

The reflexive pronoun's accusative, sik, is contracted and suffixed to the verb as -k, -sk, or -zk in order to form the reflexive suffix.^[12][13] This suffix is often referred to as Old Norse's "middle voice." In the early 13th century, the suffixes became -z and -s, and later -zt and -zst.^[12] As a middle voice, it can be thought of as passivizing an action without using passive voice syntax. This usage of reflexivity is paralleled in English with sentence pairs such as "he sat down" and "he sat himself down."

Strong nouns

This section needs expansion with: PGmc/PIE roots for stem classes; Masc. & fem. i and r tables.. You can help by adding to it. (December 2009)

Old Norse has 2 strong neuter declensions, and 3 strong masculine and feminine declensions. The masculine and feminine declensions may be referred to as the a, i, and r declensions, after their nominative plural inflections of -ar, -ir, and -r, respectively.

Though the a-declension masculines tend towards a genitive -s, and the i- and r-declension words towards -ar, many words are found going against the norm. Grautr, skógr, and hǫfundr, for example, are a-declension nouns with -ar for a genitive singular. The -i of the dative singular is frequently dropped from many words, particularly in the i-declension. Bisyllabic proper names in -arr (Einarr) or -urr (Gizurr) do not contract as hamarr does before an inflectional syllable, due to differing etymologies. The following words demonstrate two PIE o-stem reflexes, one bisyllabic, a yo-stem reflex, and an iyo-stem reflex.^[11] The latter stem type consists mainly of poetic words.^{[cv 13]}

Among the i-declension masculines are the wa-stem reflexes. These have a u-umlauted root caused by a radical v.

The strong feminines descend from PIE ā stems.^[15] PIE -ā developed into PGmc. -ō and finally Proto-Norse -u,^[14] leading to the singulars of these words being u-umlauted under that inflection's influence. Their plurals are the same as those of the analogous masculine declension, except for the nominative and accusative being the same. The ending -ir of the i-stems, as in tíð - tíðir, while not exemplified in the table below, is in fact more common in feminines than the ending of ō-stems -ar, and has become increasingly so by analogy; thus the example gjǫf in the table in later texts replaces its old plural form gjafar with gjafir. A dative singular u inflection is found in some nouns, most noticeably in the i-declension where it is sometimes accompanied by an accusative singular u.^{[cv 13]} Some nouns, prominently among nouns in the same declension as ęrmr, carry a nominative singular r inflection;^{[cv 13]} surprisingly, these are the descendants of the Proto-Germanic ijō-stems (i.e. long jō-stems, as opposed to the short counterpart seen below in the example Hęl), which historically lacked that ending; it seems that they have acquired it through confusion with the i-stems such as tíð, which did originally have it. Conversely, the i-stems have lost the nominative -r ending through analogy with ō-stem feminines.^[16] Finally, many nouns do not have any inflectional distinctions among the singulars except from the genitive. Under these circumstances the case system aligns with that of most English nouns, e.g.: "A mouse's (G) mouse (N) gave a mouse (A) to a mouse (D)." or Latin neuter u-stems (with the additional ablative and vocative cases also being identical). e.g.: "Cornūs (G) cornū (N), quod cornū (D) ērat, cornū (Ac) cum cornū (Ab) frēgit. Cornū (V)!" ("A horn's horn, that belonged to a horn, broke a horn with a horn. O horn!")

The neuter declensions' genitive and dative mirror the masculine a declension's.^[14] The nom./acc. plural is u-umlauted from the singulars, but this only alters nouns with a as their root, leaving number indistinct in many strong neuters for these cases. PGmc -ja stem reflexes, such as nęs and klæði, are umlauted without regard to stem weight.^[17]

Weak nouns

One main feature of weak nouns is that they do not distinguish the non-nominative singular cases from each other. This effectively forms a nominative-oblique case dynamic confined to the weak singulars. Historically, the Proto-Germanic weak nouns were characterized by a nasal suffix applied as a case marker. These were mostly absorbed by their preceding vowels by the time Old Norse developed, with the main exceptions being those suffixes in the weak feminine and neuter declensions' genitive plurals.^{[cv 16]} As a result, weak nouns are referred to as the n stems, a consonant stem class.

The plural inflection of the weak masculine declension is that same as the strong masculine a declension. The weak declension contains the endings -ingi and -yrki/-virki, as well as some weak versions of strong masculine nouns, names, and endings.^{[cv 16]}

The weak feminines with the -a ending vary greatly in the genitive plural, but most fall into a few groups: Nouns with -na as ending; nouns with no genitive plural; nouns that form the genitive plural by attaching the definite article's genitive plural to the nominative singular; nouns whose genitive singular is used collectively.^{[cv 17]}

As the nominative of neuter words is also the accusative, and as weak nouns have the same dative and genitive as accusative in the singulars, all of the singular forms are the same for the weak neuters. One subset of the neuter declension contains 6 nouns for parts of the body. Another contains words for objects, forming no genitive plural.^{[cv 18]}

The Indeclinable Feminines are an additional class of weak noun which originally had a word stem with the ending -in of which in Old Norse only the -i remains. They are conceptual in meaning, and because of this have no plurals and do not differentiate case.^{[cv 19]} The word lygi^[18] "lie" however has a plural. They may, in charts, be included with the feminines in -a, in which case said chart becomes:

Pronouns

The interrogatives include hvat "what", hví "why", and hvess "what sort", derived from þat, hvar "where" and hveim "whom", derived from þar, hvárt "which of two, each," and hvęrt, "whether, which of many."

There are two relative particles, er or es and sem, which can also be used as relative pronouns or adverbs. Both are completely indeclinable. The former carries the relative (non-interrogative) senses of the words which, who, when, where, and that. The latter corresponds to as, as if, alike, same as, or about the same time as, and may take on any function of er as well.

Some pronouns, such as hvárr,^[20] hvęrt,^[20] nekkvęrt,^{[cv 20]} and sá,^[21] have adjectival function. This usage generally requires a different translation than their pronominal one.

Adjectives

The comparative and superlative forms are formed by inserting -r- and -st- or -ar- and -ast- between the uninflected form of the adjective and a strong or weak ending.^{[cv 5]} In the strong adjectives, the definite and superlative are strong when indefinite, weak when definite. The comparatives are weak when both definite and indefinite, and are declined like the active participle. Some strong adjectives i-umlaut their root vowel in their comparatives and superlatives, so that stórt hús (a large house) becomes stœrst (a house most large). The past participles of weak verbs decline as strong adjectives.

Strong declension

Jarpt demonstrates the general case for declension. Gótt displays dental assimilation, while nekkvęrt demonstrates pronominal declension. The comparative and superlative follow the weak declension.

Weak declension and weak inflection of active participles

The singulars of the weak adjectival declension are modelled after those of the weak noun declensions (-an- and -on-stems), and likewise have a nominative-oblique case dynamic.

The active participle and comparative only have a weak declension and are in masculine and neuter modelled after the weak nouns with roots in -an-stems, but in feminine and plural modelled after the weak nouns with roots in -in-stems. The comparative form as in Latin, other Germanic languages etc. is also corresponding to the agent noun-ending, which in Old Norse has a weak declension with roots in -an-stem inflection as well.

The plurals are not distinguished in gender, nor in case except the dative.

Indeclinable

The indeclinable adjectives end in -i, -a or -s. They are not comparable. They originated either from regular weak adjectives, with the different endings marking gender; or adjectives with a noun ending in genitive plural or singular, as they originally were nouns.

Numbers

Eitt (one) follows the pronominal declension, and hundrað is a strong neuter noun.

Tvau, bæði, þrjú, and fjǫgur have only plural, and their declension is given below. The background of the inflection tveimr and þrimr in the dative case as seen below is the Proto-Germanic noun and adjective dative and instrumental plural ending -maz and -miz. Those endings are still used in contemporary Icelandic today. All other cardinal numbers are indeclinable.

The distributives and multiplicatives are all strong adjectives. The ordinals are weak, except for annat "the second", which is strong and fyrst "the first", which can be both.