Bod or Y?

I can’t get my around when I should use ‘bod’ or ‘y’ for ‘that’ in a sentence. I know ‘y’ is not commonly used in speech, but it must be used when writing. Some example sentences would also help

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In short, you use bod when the subordinate clause
written on its own would start with either a present tense or perfect form of to be, and y(r) if it isn’t. (This is a gross oversimplification of the matter, as it completely disregards focused sentences, for example)

Let’s shine a light on this with two examples:
I know that you are going now.Dw i’n gwybod fod ti’n mynd nawr.
I think we should stay.Dw i’n meddwl (y) dylen ni aros.

One of the problems here is that the formation of complex sentences in Welsh is rather complicated. So you can either try to pick it up naturally by reading a lot and sort of notice and remember how it is done, or you go down the road of studying grammar books (like any of Gareth King’s books, I can heartily recommend them).