Could this also be translated as "I will pay"? I took it to be a short form of [fe] dala[f] i. But got marked as wrong.
No. In this instance 'I will pay' is Tala i. If you add Fe/Mi to the beginning of the sentence, there is a mutation. In that case, 'I will catch' becomes Fe ddala i, while 'I will pay' becomes Fe dala i. D mutates into Dd. T mutates into D.
Yes, but I thought it is common to leave out fe/mi but to keep the mutation as if it were present?
For example, King talks about a tendency to mutate whenever an inflected verb is used (A Comprehensive Grammar, §11d).
It is particularly common with the ges i, gest ti, gaeth e, ... forms of cael (instead of ces i, ...) but not so common in its cefais i, cefaist ti, cafodd e, ... forms.
While starting to learn, it might be easier to avoid mutating affirmative verbs, other than those particular cael forms, unless explicitly using mi/fe in front of them.
That's what I thought too. I'm sure I see instances of such mutation with fe/mi omitted in tweets from native Welsh speakers