"Dych chi wedi blino?"
Translation:Are you tired?
No. See the course notes (https://forum.duolingo.com/comment/17638579).
Welsh ti and chi are very similar to French 'tu' and 'vous'. Nothing to do with dialect.
ti is used with an individual with whom you are on familiar terms. chi is used with any two or more people or with an individual with whom you are not on familiar terms.
'n/yn and wedi are not used together. See the earlier comment.
Welsh is not "English with funny words" -- it has its own way of expressing things.
Sometimes, Welsh uses two words to express something that English would use one word for; sometimes, Welsh uses one word to express something that English would use two words for.
The word order may be different. There may be words which are needed in English that Welsh does not need (e.g. "do" in questions), or there may be words which are needed in Welsh that English does not need (e.g. yn linking two verbs). And so on.