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 say, doesn’t this make the sentence “how to to say”? Sometimes when there is a string of such words, it seems there is no i between them, e.g. “hoffi gweithio” not “hoffi i gweithio”. Confused. Sorry.
Sometimes it is there because it signifies ‘in order to’ as in barod i gysgu - ready (in order to) sleep. Other times it is there because it is part of the proceeding verb’s pattern as in mynd i gysgu - mynd is one of the verbs that uses a +i pattern. As with all rules, there are exceptions which only add to the confusion!
The trick is not to try and translate literally back into English but to absorb the ‘when and where’ it is used by listening practice.
the problem here is that the English “to” has different meanings. If it marks only the infinitiv of a verb, like : I want to do , I like to eat etc. you don’t need the “i” in Welsh.
But somettimes “to” shows you something like a direction, then you use the Welsh “i”
It is normally the word before the “i” that causes the use of it, so it’s maybe to easier to learn:
I’m going to do something Dw i’n mynd i wneud
How to do it Sut i wneud o
I want to do something. Dw i isho gwneud
I’m ready to sleep Dw i’n barod i gysgu.
And as always, don’t worry too much about it. You’ll be understood anyway.
I think of it as “in order to” - so; sut i ddweud (how i’n order to say)
Hoffi gweithio - Like to work.
In addition to what the others have already said, you are falling into the trap of trying to map Welsh to English one-on-one. In Welsh, an unconjugated verb in its basic form is always also a noun, so “meddwl” and “dweud” are verbnouns, Trying to map that to the English infinitive form to think and to say is only one aspect of the verbnoun. Another way to use the verbnoun is to express sentences like “Learning Welsh is easy.” -> "Mae dysgu cymraeg yn hawdd."