Formal:
* Diolch yn fawr, fy ffrind. (This is the most literal translation and is very formal.)
* Diolch yn fawr iawn, fy ffrind. (This adds emphasis and makes it slightly less formal.)
Informal:
* Diolch, fy ffrind. (This is a simple and friendly way to say thank you.)
* Diolch i ti, fy ffrind. (This means "thank you to you" and is more personal.)
More familiar:
* Diolch yn fawr iawn, cariad. (This means "thank you very much, love" and is suitable for a close friend.)
* Diolch yn fawr iawn, bach. (This means "thank you very much, little one" and is suitable for a close friend you are affectionate with.)
You can also add a specific reason for your gratitude, such as:
* Diolch yn fawr iawn am eich cymorth. (Thank you very much for your help.)
* Diolch yn fawr iawn am y rhodd. (Thank you very much for the gift.)
The best way to choose the right phrase depends on your relationship with your friend and the situation.