"Yn" is an untranslateable verbal particle which is used to connect a form of "Bod" (To be) to a noun, verb-noun or adjective. I.e in this isntance "Dych" is the second personal formal/plural conjugation of "Bod" which needs to be connected to the verb-noun "gwneud" using "yn" or as it becomes in this instance "'n". There are a couple of sentences where you would expect it to be used but it isn't. These sentences include the words "Eisiau/Isio" and "Angen". This is because whilst we use them to mean "To want" and "To need" respectively they are not actually verbs. e.g "Dych chi eisiau/isio/angen" not "Dych chi'n eisiau/isio/angen". Also it's important to note that "yn" also can mean "in" when talking about a specific place.

Обожаю этот язык, ну как можно его не любить?