"I'm going to America after retiring."
Translation:Dw i'n mynd i America ar ôl ymddeol.
You need to use yn or 'n to link the verb "to be" with the verb-noun that follows. So it's dw i'n mynd -- I am going/I go dw i'n dod -- I am coming/I come dw i'n darllen -- I am reading/I read dw i'n ysgrifennu -- I am writing/I write You can use wedi instead of yn to put the sentence into the past tense dw i wedi mynd -- I went (== Es i in short form) dw i wedi dod -- I came (== Des i) dw i wedi darllen -- I read (== Darllenais i) dw i wedi ysgrifennu -- I wrote (== Ysgrifennais i) You also need "yn" to join the verb to be to a complement - Mae e'n wyrdd -- he is green. There's soft mutation in this case where an adjective follows yn. There no mutation when a verb-noun follows yn.
There are only one or two exceptions. They are want and need. So Dw i eisiau -- I want Dw i angen -- I need These seem odd to me as a learner. I think they may have emerged in spoken Welsh from a different pattern. When I was in school 40+ years ago, we learnt Mae eisiau arna i -- I want, lit. "there is want on me" but that doesn't seem to be common now.