"I am going to drink beer at twenty minutes to four."
Translation:Dw i'n mynd i yfed cwrw am ugain munud i bedwar.
Don't worry too much about mutations I am fluent in welsh and have seen since I was six (I am sixteen now) and I'm only just getting them. This page has a wonderful section on mutations if you do want to focus on them (https://en.wikipedia.org/wiki/Literary_Welsh_morphology)
Duolingo teaches 'colloquial' Welsh more than 'literary' (literary Welsh is something akin to Shakespearean English, and more for poetry and things like that). So here's the page for colloquial Welsh, although by all means look at literary as well! https://en.wikipedia.org/wiki/Colloquial_Welsh_morphology
Something that can make mutations easier is remembering that most prepositions cause the soft mutation - lack of mutation is not as common. The nasal and the aspirate mutations are rarer and their contexts can be learnt quite easily.
Nasal: after fy (my), after yn (in), and after an- ('un-' as in 'unhappy')
Aspirate: after ei (her). Also happens less consistently after a (and), after â (with), after gyda / efo (with) - and for masculine nouns after tri (three), chwe (six - special pronunciation in this context!).