"Who are you?"

Translation: Pwy dych chi?

[CharredCheese]

Pwy wyt ti?

[Rae.F]

They've added information to the Tips and Notes:

There are two forms of 'you' in Cymraeg, similar to other modern languages, being chi and ti. Chi is used with a group and people to whom you'd like to show respect. This is taught in this unit and ti later on. The form dych chi will change its meaning to you (are/do) to are/do you? when used with a question, much like in English.

That would mean "Pwy dych chi?" is for plural or formal and "Pwy wyt ti?" is for singular or informal. French does the same thing.

[kaememerel]

OK, so what's the story with "dach" - just a variant of "dych" ? Cause it looks like a typo.

[IeuanHywel]

North Wales say 'dach' south say 'dych' and mid say 'ydych

[ipw133]

People you don't know = dych chi. You are unlikely to ask someone you know, who they are.

[ipw133]

You need to understand the difference between formal and informal. That is common in many languages. (Tu vs Vous) etc. Wyt ti (someone you know) Dych chi (older person or a new acquaintance).