Use of "i" before an infinitive

I am confused when “i” is used before an infinitive. I’m sure there’s a rule but at the moment it seems a bit random to me.
For example “what to do tonight” is translated as “beth i gwneud heno” but there have been several instances when “i” has not been used before the infinitive.
Help please.

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Helo Andrew, somehow your question slipped by unnoticed and unanswered for a whole day.

Let’s get to the core of the confusion: the infinitive by itself never has an i before it. It’s always the phrase before the infinitive that sometimes ends with it. I am afraid that I can’t give you a hard and fast rule telling you when to use an i and when not, so in the end you’ll pick it up phrase by phrase.

tl;dr: Prepositions are tricky :wink:

Maybe you’ll find this older thread on the same topic useful: