The usual English word is "mutation" -- it's when the first sound of a word changes for grammatical reasons.

The most common mutation is "soft mutation"; the soft mutation of T is D.

A treiglad (They are 7!) changes the beginning of the word, to a different sound. E.g: Tost-Dost, Draig-Ddraig, Cath-Gath e.t.c You mostly use the treiglad meddwl (stated above) When speaking, and sometimes the treiglad Llosog (but don't worry about that for now.)

You use the treiglads when just speaking about the item in a short sentance. e.g: Dost a siocled > Dwi'n cael tost a siocled

Did you see that it wasn't used in the sentance? I am fluent in Welsh, so I hope this helps :) If you have any other questions, please feel free to ask, and I will be more than happy to answer them. :)