Informal:
* Ti'n siarad: You (singular, informal) speak
* Chi'n siarad: You (plural, informal) speak
Formal:
* Rydych chi'n siarad: You (singular, formal) speak
* Rydych chi'n siarad: You (plural, formal) speak
Note: In formal situations, it is always best to use the formal form, even when addressing one person.