Sorry if this has been answered a million times - I just joined the forum, and can’t see the wood for the trees…
Is there a rule to tell you why it’s "wyt ti’n barod i siarad?’ but ‘wyt ti isio aros?’
Hi Bronwen, and welcome to the forum! 
This is a distinction which doesn’t map easily to English, so it’s hard to deal with on a rule-based approach. The easiest short-cut to it would probably be to think in terms of the word before the ‘i’, rather than the verb after it - so ‘barod+i’, ‘mynd+i’, and a few others.
On a longer term basis, you’ll find that not worrying too much about this kind of stuff, and just letting the increased exposure you’ll get to certain patterns bed them in until it starts to become natural, is a good way to fine-tune your language production
Once you’ve heard ‘mynd i wneud o’ often enough, saying ‘mynd gwneud’ will just sound wrong to you
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Waw! That was quick!
I’m turning off my panic button from now on…
Diolch yn fawr
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Excellent! Panic buttons are at their best when turned off…
cmc has a ‘cheat sheet’ for this topic at: http://clwbmalucachu.co.uk/cmc/cheat/cheat_vn_no_i.htm
You will notice that these verbs don’t have any action/movement involved eg eisiau is all ‘in your head’.
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Sorry - I meant to say thanks for this. I’m trying to learn these verbs through practice, as advised, but I’ve kept your helpful crib sheet for reference!