"I like coffee and chocolate."
Translation:Dw i'n hoffi coffi a siocled.
It normally means "in the". But here it's an untranslatable grammatical particle used with mae: https://en.wiktionary.org/wiki/yn#Etymology_1_2
It may help to break the sentence up a bit: dw i is "I am" (or rather, "am I"), so this sentence is literally "am I liking [of] coffee".
The liking "belongs" to the coffee, so "I like him" would be dw i'n ei hoffi - "am I his liking".
The present tense in general tends to be expressed as dw i'n [verbal noun].
The short version: with eisiau (isio) and angen, use dw i.
With all other verbs (such as hoffi "to like", yfed "to drink" etc.), use dw i'n.
The yn (abbreviated here as 'n) is an untranslatable linking particle that is needed with verbs, but eisiau and angen aren't "proper" verbs - for historical reasons related to how such expressions came about, they don't use the yn so you will see simply dw i eisiau afal etc. -- but e.g. dw i'n bwyta afal for "I am eating an apple".