Prynhawn da, i chi gyd.
I’m currently on ‘lesson 5’ of SSiW and have become confused by something.
Throughout the lesson it is taught that;
Mae’n = it is
I think = dwi’n meddwl
Mae’n diddorol (as the example is given)
So why does the example sentence ‘I think it’s interesting’ then become: ‘Dwi’n meddwl bod hi’n diddorol’.
My instinct as a learner would think the sentence would simply be: ‘Dwi’n meddwl mae’n diddorol’.
Help! Diolch 
It’s because in Welsh we have to put the ‘that’ in where in English we often leave it out - “I think that it is interesting”, and in this construction, “that it is” = “bod hi’n”.
Diolch yn fawr iawn, Siaron! Very helpful!
Next question…where does “hi’n” come from in this example. Why does it replace “mae’n”?
“hi” is “it” when you’re talking about something with feminine gender. “mae’n” is an abbreviation of either “mae hi’n” or “mae fe’n/mae fo’n” but when we use the ‘that’, we can’t abbreviate because ‘bod’ doesn’t end in a vowel, so it has to be the full “bod hi’n” (or “bod fe’n/bod fo’n” when talking about something of masculine gender).
Siaron you are the Grammar Whisperer!! Every time one of us has a question like this you always give the clearest, most easily understood answers!
DIOLCH YN FAWR IAWN indeed!
You’re too kind!
… doesn’t mean I don’t check Gareth’s ‘Modern Welsh’ too sometimes to make sure though! (he’s the real Grammar Whisperer!) 
Diolch