There are different rules for each of the mutations, and whilst they only change certain letters, the mutations change them differently. Below are the 5 mutations that you must be able to recognise and use by the time that your exams arrive.

Mutation non-negotiables

1. Treiglad Meddal on adjectives following "yn" (including "yn" in phrases like "achos mae'n ..."), 

     eg. Mae rygbi yn wych.

2. Treiglad Meddal on words following "am", "ar", "i" and "o", meaning "at", "on", "to" and "from", 

     eg. Am bedwar o'r gloch ar Ddydd Gwener, rydw i'n mynd o Ferthyr Tudful i Gaerdydd. 

3. Treiglad Meddal on words following "nos", meaning "night", 

      eg. nos Lun, nos Fawrth, nos Fercher, Nos Wener.

4. Treiglad Trwynol on words following "fy", meaning "my", 

       eg. fy nheulu, fy mrawd, fy mag, fy ngwaith cartref, fy mhensil, fy nyddiadur

5. Treiglad Trwynol on place names following "yn" meaning "in", especially those beginning with the letter C, 

     eg. yng Nghaerffili, yng Nghaerdydd, yng Nghasnewydd, yng Nghaernarfon, yng Nghymru

Once you're confident with the five above, there are a few other important mutations to be able to recognise. Unlike the list above, you will not be expected to be able to use all of these during your exams.

1. Treiglad Meddal on words following "dy" meaning "your", 

     eg. dy frawd, dy fam

2. Treiglad Meddal on words following "ei" meaning "his", 

  eg. ei frawd, ei fam

3. Treiglad Meddal on words following the positive form of 3 of the Conditional Tenses - "Hoffwn i", meaning "I would like", "Dylwn i", meaning "I should" and "Gallwn i", meaning "I could",  eg. Hoffwn i fynd, Dylwn i wrando, Gallwn i fwyta

4. Treiglad Meddal on words following "hoff", meaning "favourite" and "cas" meaning "worst/least favourite", 

      eg. hoff bwnc, cas raglen

5. Treiglad Meddal on words following "at", "dan", "hyd", "dros", "drwy", "gan", "heb" and "wrth", meaning "at", "on", "to" and "from",    eg. Am bedwar o'r gloch ar Ddydd Gwener, rydw i'n mynd o Ferthyr Tudful i Gaerdydd. 

6. Treiglad Llaes on words following "ei" meaning "her", eg. ei theulu, ei chi

7. Treiglad Llaes on words following "a" meaning "and", eg. ci a chath, ffrindiau a theulu, sglodion a physgod

8. Treiglad Llaes on words following "gyda" and "â", meaning "with",  eg. gyda phobl, ymweld â theulu

9. Treiglad Llaes on words following "tua" meaning "about/roughly", eg. tua thri o'r gloch, tua phump person