This is explained in the Notes for 'Wanting3'.
In common with many languages, Welsh has two forms of 'you':
- ti - singular 'you' only, and only used with people with whom you are on familiar terms.
- chi - always used when speaking to more than one person, and also used for individuals with whom you are not on familiar terms.
Welsh verbs have different forms for use with ti and with chi, such as rwyt ti and dych chi - but both mean 'you are'.
Yes, in the context of a sentence. That is explained in the course notes (https://forum.duolingo.com/comment/17638579), for example in the sections 'Clothes' (https://www.duolingo.com/skill/cy/Dillad1/tips-and-notes) and 'Wanting' (https://www.duolingo.com/skill/cy/Wanting/tips-and-notes).