The morphology of the Welsh language shows many characteristics perhaps unfamiliar to speakers of English or continental European languages like French or German, but has much in common with the other modern Insular Celtic languages: Irish, Scottish Gaelic, Manx, Cornish, and Breton. Welsh is a moderately inflected language. Verbs conjugate for person, tense and mood with affirmative, interrogative and negative conjugations of some verbs. A majority of prepositions inflect for person and number. There are few case inflections in Literary Welsh, being confined to certain pronouns.

Modern Welsh can be written in two varieties – Colloquial Welsh or Literary Welsh. The grammar described on this article is for Literary Welsh.

Initial consonant mutation is a phenomenon common to all Insular Celtic languages (there is no evidence of it in the ancient Continental Celtic languages of the early first millennium). The first consonant of a word in Welsh may change depending on grammatical context (such as when the grammatical object directly follows the grammatical subject), when preceded by certain words, e.g. i, yn, and a or when the normal word order of a sentence is changed, e.g. Y mae tŷ gennyf, Y mae gennyf dŷ "I have a house".

Welsh has three mutations across four paradigms: the soft mutation (Welsh: treiglad meddal), the nasal mutation (Welsh: treiglad trwynol), and the aspirate (or spirant) mutation (Welsh: treiglad llaes); and the mixed mutation (Welsh: treiglad cymysg) where the aspirate mutation is applied where possible, else the soft mutation is applied. These are represented in writing, as shown in the table below along with their corresponding IPA symbols.

A blank cell indicates no change.

For example, the word for "stone" is carreg, but "the stone" is y garreg (soft mutation), "my stone" is fy ngharreg (nasal mutation) and "her stone" is ei charreg (aspirate mutation).

* The soft mutation for g is the simple deletion of the initial sound. For example, gardd "garden" becomes yr ardd "the garden". But this can behave as a consonant under certain circumstances, e.g. "gellir" (one can) becomes "ni ellir" (one cannot) not "*nid ellir". This however also stands alongside gardd ('a garden'), but yr ardd ('the garden') not *y ardd.

Welsh has no indefinite article. The definite article, which precedes the words it modifies and whose usage differs little from that of English, has the forms y, yr, and ’r. The rules governing their usage are:

1. When the previous word ends in a vowel, regardless of the quality of the word following, ’r is used, e.g. mae'r gath tu allan ("the cat is outside"). This rule takes precedence over the other two below.
2. When the word begins with a vowel, yr is used, e.g. yr arth "the bear".
3. In all other places, y is used, e.g. y bachgen ("the boy").

The letter w represents both a consonant /w/ and the vowels /u/ and /ʊ/ - a preceding definite article will reflect this by following the rules above, e.g. y wal /ə ˈwal/ "the wall" but yr wy /ər ˈʊˑɨ/ or /ər ˈʊi/ "the egg". However, pre-vocalic yr is used before both the consonantal and vocalic values represented by i, e.g. yr iâr /ər ˈjaːr/ "the hen" and yr ing /ər ˈiŋ/ "the anguish". It is also always used before the consonant h, e.g. yr haf /ər ˈhaːv/ "the summer". The first rule may be applied with greater or less frequency in various literary contexts. For example, poetry might use ’r more often to help with metre, e.g. ’R un nerth sydd yn fy Nuw "The same power is in my God" from a hymn by William Williams Pantycelyn. On the other hand, sometimes its use is more restricted in very formal contexts, e.g. Wele, dyma y rhai annuwiol "Behold, these are the ungodly" in Psalm 73.12.

The article triggers the soft mutation when it is used with feminine singular nouns, e.g. tywysoges "(a) princess" becomes y dywysoges "the princess", but no mutation in the plural: y tywysogesau "the princesses".

Like most Indo-European languages, all nouns belong to a certain grammatical gender; in this case, masculine or feminine. A noun's gender conforms to its referent's natural gender when it has one, e.g. mam "mother" is feminine. There are also semantic, morphological and phonological clues to help determine a noun's gender, e.g. llaeth "milk" is masculine as are all liquids, priodas "wedding" is feminine because it ends in the suffix -as, and theatr "theatre" is feminine because the stressed vowel is an e. Many everyday nouns, however, possess no such clues.

Sometimes a noun's gender may vary depending on meaning, for example gwaith when masculine means "work", but when feminine, it means "occasion, time". The words for languages behave like feminine nouns (i.e. mutate) after the article, e.g. y Gymraeg "the Welsh language", but as masculine nouns (i.e. without mutation of an adjective) when qualified, e.g. Cymraeg da "good Welsh". The gender of some nouns depends on a user's dialect, and although in the literary language there is some standardization, some genders remain unstable, e.g. tudalen "page".

Welsh has two systems of grammatical number. Singular/plural nouns correspond to the singular/plural number system of English, although unlike English, Welsh noun plurals are unpredictable and formed in several ways. Some nouns form the plural with an ending (usually -au), e.g. tad and tadau. Others form the plural through vowel change, e.g. bachgen and bechgyn. Still others form their plurals through some combination of the two, e.g. chwaer and chwiorydd.

Several nouns have two plural forms, e.g. the plural of stori "story" is either storïau or straeon. This can help distinguish meaning in some cases, e.g. whereas llwyth means both "tribe" and "load", llwythau means "tribes" and llwythi means "loads".

The other system of grammatical number is the collective/singulative. The nouns in this system form the singulative by adding the suffix -yn (for masculine nouns) or -en (for feminine nouns) to the collective noun. Most nouns which belong in this system are frequently found in groups, for example, plant "children" and plentyn "a child", or coed "trees, forest" and coeden "a tree", ffawydd "a beechwood, beech trees, beeches" and ffawydden "a beech tree". In dictionaries, the collective form, being the root form, is given first.

Adjectives normally follow the noun they qualify, e.g. mab ieuanc "(a) young son", while a small number precede it, usually causing soft mutation, e.g. hen fab "(an) old son". The position of an adjective may even determine its meaning, e.g. mab unig "(a) lonely son" as opposed to unig fab "(an) only son". In poetry, however, and to a lesser extent in prose, most adjectives may occur before the noun they modify, but this is a literary device.^[1] It is also seen in some place names, such as Harlech (hardd + llech)^[2] and Glaslyn.

When modifying a noun (i.e. in an attributive construction) belonging to the feminine, adjectives undergo soft mutation, for example, bach "small" and following the masculine noun bwrdd and the feminine noun bord, both meaning "table":

For the most part, adjectives are uninflected, though there are a few with distinct masculine/feminine and/or singular/plural forms. A feminine adjective is formed from a masculine by means of vowel change, usually "w" to "o" (e.g. crwn "round" to cron) or "y" to "e" (e.g. gwyn "white" to gwen). A plural adjective may employ vowel change (e.g. marw "dead" to meirw), take a plural ending (e.g. coch "red" to cochion) or both (e.g. glas "blue, green" to gleision).

Adjective comparison in Welsh is fairly similar to the English system except that there is an additional degree, the equative (Welsh y radd gyfartal). Native adjectives with one or two syllables usually receive the endings -ed "as/so" (preceded by the word cyn in a sentence, which causes a soft mutation except with ll and rh: cyn/mor daled â chawr, "as tall as a giant"), -ach "-er" and -af "-est". The stem of the adjective may also be modified when inflected, including by provection, where final or near-final b, d, g become p, t, c respectively.

Generally, adjectives with two or more syllables use a different system, whereby the adjective is preceded by the words mor "as/so" (which causes a soft mutation except with ll and rh), mwy "more" and mwyaf "most".

The literary language tends to prefer the use inflected adjectives where possible.

There are also a number of irregular adjectives.

These are the possessive adjectives:

The possessive adjectives precede the noun they qualify, which is sometimes followed by the corresponding form of the personal pronoun, especially to emphasize the possessor, e.g. fy mara i "my bread", dy fara di "your bread", ei fara ef "his bread" etc.

Ein, eu and feminine ei add an h a following word beginning with a vowel, e.g. enw "name", ei henw "her name".

The demonstrative adjectives are inflected for gender and number:

These follow the noun they qualify, which also takes the article. For example, the masculine word llyfr "book" becomes y llyfr hwn "this book", y llyfr hwnnw "that book", y llyfrau hyn "these books" and y llyfrau hynny "those books".

In literary Welsh, far less use is made of auxiliary verbs than in its colloquial counterpart. Instead conjugated forms of verbs are common. Most distinctively, the non-past tense is used for the present as well as the future.

The preterite, non-past (present-future), and imperfect (conditional) tenses have forms that are somewhat similar to colloquial Welsh, demonstrated here with talu 'pay'. There is a regular affection of the a to e before the endings -ais, -aist, -i, -ir and -id.

To these, the literary language adds pluperfect, subjunctive, and imperative forms with affection before -wyf and -wch.

In Welsh, prepositions frequently change their form when followed by a pronoun. These are known as inflected prepositions. They fall into three main conjugations.

Firstly those in -a- (at, am (stem: amdan-), ar, tan/dan):

Secondly those in -o- (er, heb, rhag, rhwng (stem: rhyng-), tros/dros, trwy/drwy (stem: trw-/drw-), o (stem: ohon-), yn). All apart from o add a linking element in the third person (usually -dd-, but -ydd- in the case of trwy/drwy, and -t- in the case of tros/dros):

Thirdly, those in -y- (gan and wrth). Gan includes both vowel changes and a linking element, while wrth has neither:

Finally, the preposition i is highly irregular:

All inflected prepositions may optionally be followed by the appropriate personal pronouns, apart from i, where this is only possible in the third person, thanks to its proper endings in the other persons sounding the same as the pronouns. In slightly less formal Welsh, the endings are split off the first and second persons of i to be interpreted as pronouns instead, although this creates the anomalous pronoun mi.

The majority of prepositions (am, ar, at, gan, heb, hyd, i, o, tan/dan, tros/dros, trwy/drwy, wrth) trigger the soft mutation. The exceptions are â, gyda, and tua, which cause the aspirate mutation; yn, which causes the nasal mutation; and cyn, ger, mewn, rhag, and rhwng, which do not cause any mutation.