Formal:
* Rwy'n dy garu di. (Roo-in dih car-oo dih) This is a literal translation and is the most formal way to say it.
Informal:
* Dw i'n dy garu di. (Dwoo ee'n dih car-oo dih) This is a more common and casual way to say it.
* Caru 'r ti. (Car-oo er tee) This is a more informal and affectionate way to say it. It literally translates to "Love you".
Other options:
* Cariad. (Car-ee-ad) This means "Love" and can be used as a term of endearment.
* Rwy'n dy garu. (Roo-in dih car-oo) This is a shorter, more informal version of "Rwy'n dy garu di."
The best option for you will depend on the specific situation and the level of formality you wish to convey.