The 'a' causes an aspirate mutation. https://www.bbc.co.uk/bitesize/guides/zsv8wmn/revision/3
chi means "you" (when speaking politely and/or to more than one person).
i in that context means "I".
The 'n in both cases is a contracted form of the particle yn, which doesn't get translated.
- dw i'n hoffi coffi "I like coffee"
- rwyt ti'n hoffi coffi "you [one person] like coffee"
- mae e'n hoffi coffi "he likes coffee"
- mae hi'n hoffi coffi "she likes coffee"
- dyn ni'n hoffi coffi "we like coffee"
- dych chi'n hoffi coffi "you [polite/several people] like coffee"
- maen nhw'n hoffi coffi "they like coffee"
So you not only need the right subject pronoun (e.g. i for "I", chi for "you") but also the correct verb form (e.g. dw i but dych chi).