Dych chi = 'You do' or "do you' depending on whether it's a question or a statement.
For almost every verb except eisiau you need to add 'n to link to the verb
dych chi'n hoffi coffi? = 'Do you like coffee?
dych chi'n yfed coffi? = 'Do you drink coffee?
dych chi eisiau coffi? = 'Do you want coffee?