Welsh, as with all other Celtic languages, often sees changes made to the beginning of words depending on the word that precedes it, or the role it plays in the sentence. These changes are known as "mutations", of which Welsh has three distinct types. Common situations in which a mutation may occur are when a word follows a preposition, possessive, or number.
The three classes of mutation are soft, nasal, and aspirate. Soft mutations are by far the most common, whereas aspirate mutations are becoming applied less and less in the spoken language. All mutations occur to words commencing with one of eight different consonants (p,t,c,b,d,g,m,ll,rh), replacing them with different consonants according to mutation type. There are two important things to note at this point:
- ll and rh are treated as separate letters in their own right and hence have their own section in the dictionary
- A good knowledge of the possible changes is essential when searching for a word in the dictionary: botel, mhotel, and photel – for example – are all mutated forms of the "base" word potel (meaning "bottle"), which will only be listed in this base (or radical) form in the dictionary. Similarly, knowing that the soft mutation of an initial g causes it to disappear will help to trace an instance of orsaf back to its radical form gorsaf ("station").
Soft mutation 
Soft mutation occurs in several common situations in the language. Here is a list of those situations.
1. Feminine nouns: Welsh has two genders of noun but the importance of committing these genders to memory is slightly less obvious when teaching oneself then it is in other languages such as French and Spanish. While in French gender dictates a series of changes - which definite article is used (le/la), how adjectives agree (petit/petite)... - in Welsh the gender of the noun has a more subtle, but important role in constructing grammatically correct sentences.
Inserting the definite article y (yr before a vowel) causes a feminine noun to undergo a soft mutation.
- bachgen - boy
- merch - girl
after adding the definite article we have:
- y bachgen - the boy
- y ferch - the girl (As can be seen from the table above, soft mutation means m becomes f )
The same occurs when the number un - one is placed precedes the noun. Hence:
- un bachgen - one boy
- un ferch - one girl.
Note 'a boy' would be put into Welsh as simply bachgen, there is no equivalent for the indefinite article 'a' in Welsh.
2. Adjectives following feminine nouns Continuing to use the examples above we will see how any adjective following a feminine noun will also undergo soft mutation.
- y bachgen - the boy
- y ferch - the girl
Adding the adjective bach (Eng. little) gives us:
- y bachgen bach - the little boy
- y ferch fach - the little girl ( b also mutates to f )
The difference is clear in the following two phrases:
- Prynhawn da - Good afternoon
- Noswaith dda - Good evening
Noswaith (evening) mutates the adjective da (good)→dda because it is feminine, while prynhawn (afternoon) - a masculine noun - does not.
3. After the majority of prepositions. Most prepostions, particularly monosyllabic ones, require a soft mutation.
- Croeso i Gymru - Welcome to Wales (C → G)
- O Fangor i Landudno - From Bangor to Llandudno (B → F and Ll → L)
List of prepositions triggering a soft mutation -
am : ar : at : dan : dros : drwy : gan : heb : hyd : i : o : wrth
|Welsh||English||base form of mutated word|
|am flynyddoedd||for years||blynyddoedd|
|ar daith||on a journey||taith|
|at gost o £15||at a cost of £15||cost|
|dan reolaeth||under control||rheolaeth|
|dros ddwy awr||over two hours||dwy|
|drwy ddull electronig||by electronic means||dull|
|gan Lywodraeth Cymru||by the Welsh Government||llywodraeth|
|heb rybydd||without warning||rhybydd|
|hyd gopa'r bryn||to the top of the hill||copa|
|i blant||for children||plant|
|o orsaf yr heddlu||from the police station||gorsaf|
|wrth greu swyddi||by creating jobs||creu|
4. After certain possessives The pronouns for the possessives 'your' and 'his' also require soft mutation. Other possessives require nasal and aspirate mutations.
dy...(di) - your (familiar)
ei...(fe/o) - his
tafarn - pub
dy dafarn (di) - your pub
llyfr - book
ei lyfr (fe/o) - his book
The pronoun ei also mean 'her', as well as his, but in this case it requires an aspirate mutation, as is seen in the section below
Aspirate Mutation 
This type of mutation is shown in the fourth column of the table.
1. After a certain possessive The pronoun ei (this time, meaning her) causes an aspirate mutation.
ei...(hi)' - her
- Carreg - Stone
- Tad - Father
After adding ei, we have:
- Ei Charreg (hi) - Her Stone
- Ei Thad (hi) - Her Father
2. After a/ac A/ac is the word and (Ac is used before a vowel, hence it cannot cause an aspirate mutation).
...a... - ...and...
- Trosedd - Crime
- Cosb - Punishment
- Bara - Bread
- Caws - Cheese
If we link two of them together with a...
- Trosedd a chosb - Crime and Punishment
- Bara a chaws - Bread and cheese
Nasal Mutation 
Nasal mutation is the third column in our table.
1. After the locative The locative in Welsh is the word yn (in).
- This should not be confused with yn when it operates as a particle in constructing a verb. When used in this grammatical sense it does not require mutation.
Yn... - In...
- Porthmadog - Porthmadog
- Tresaeth - Tresaeth
- Caerdydd - Cardiff
- Bangor - Bangor
- Dolgellau - Dolgellau
- Glanllyn - Glanllyn
If we add the locative:
- Ym Mhorthmadog - In Porthmadog
- Yn Nhresaeth - In Tresaeth
- Yng Nghaerdydd - In Cardiff
- Ym Mangor - In Bangor
- Yn Nolgellau - In Dolgellau
- Yng Nglanllyn - In Glanllyn
You may have noticed that yn also mutates. This is not a typing error. It mutates before a nasal mutation too:
- Yn - Before a mutated T or D
- Ym - Before a mutated P or B
- Yng - Before a mutated C or G
2. After fy. The word fy, meaning "my", causes nasal mutation in the next word:
- dillad - clothes
- fy nillad - my clothes
- brawd - brother
- fy mrawd - my brother
- tadau - fathers
- fy nhadau - my fathers
In the spoken language, because the nasal mutation occurs so rarely, the word fy can drop out entirely:
- nillad - my clothes
- mrawd - my brother
- nhadau - my fathers