Infinitive confusion

[Hendrik]

You’re not the first to be confused by this, nor will you be the last. Here’s a link to an older thread where this was discussed, maybe this will clear things up a bit:

To use "i" or not to use "i"

My understanding is that words like “meddwl”, “dweud” etc mean “to think”, “to say” etc. Listening closely to SSiW it seems that sometimes an i precedes these in a sentence (e.g. barod i gysgu", “sut i ddweud”, “mynd i gysgu” etc) but sometimes, in other circumstances, not. My question is, why is i sometimes needed and sometimes not and what is the rule? I can appreciate that sut means “how”, rather than “how to”, so can see some sense in adding the i but then again given that dweud means to sa…