Good afternoon/prynhawn da! 
I am currently on ‘Challenge 4’ of SSiW and have become confused.
In a previous lesson I was taught:
Bo fi = That I
Wedi bod = Been
But in ‘Challenge 4’ the following sentence confused me:
“I think that I’ve still got to practice more.”
In this example they’ve replaced ‘bo fi’ with ‘bod’.
Why is that?
Diolch!
It is because the second half of the sentence uses a structure where the ‘I’ part comes at the end…
…rhaid i fi = I have to/ got to
So you just need the ‘that’ = bod in the middle of the sentence in this situation, or there would be two 'I’s
Does that make sense?
Rich
I don’t know if you’re doing North or South, but in the North ones Aran mentions in passing that “rhaid i mi” translated literally would be “there is a must for me”.
So you’re not actually saying “I think that I’ve still got to”. You’re saying “I think that there’s still a must for me”. So the “fi” (which is the I/me part) comes at the end rather than after “bod”.
Thank you Rich! Very kind of you to reply. That makes sense, and I guess I’ll get to learn these adaptions over time!
Diolch yn fawr.
Diolch Alan! That makes sense to me now, but I have a feeling it will take a while for me to get to gribs with these kinds of changes.
hi - I was going to ask exactly the same question as the poster on here- but I’m still unsure- do you mean if you need to say something with “mae rhaid i fi” it means you use bod instead? You’d say …“bod mae rhaid i fi etc?” - I was confused in challenge 4 but when it popped up again I realised I was still confused and I’d better make sure I understand it properly - you use “bo fi” if it’s not something like “mae gyda fi” or “mae rhaid i fi etc?”
No, you wouldn’t ever say “bod mae rhaid” because mae is itself a tense of bod. In speech, you’ll often hear sentences start with the ‘rhaid’, and the preceding form of bod (mae/bydd/oedd) has dropped off and is merely implied by context.
You could, however, say things like “mae rhaid bod… (yn bwysig)” = “it must be… (important)”, and bod is used here instead of i because the following thing is an indefinite article, not a pronoun. NB, there is an implied “iddo fo” between rhaid and bod in that example because it, too, is often left out in speech.
The other bod that may be confusing you is the one that means ‘that’ - that’s when you’d use “bo fi” (that I) - and that one never starts a sentence. It could, though, come in the middle of a sentence next to rhaid - dwedodd o bod rhaid i fi ymarfer (he said that I must practise). Is that the bit that’s tripping you up, perhaps?
Does that make it a bit clearer?
hi there - yes it’s the challenge 4 one (bo fi or bod) - I don’t know what to say to complete the sentences - I got the “bo fi” at first but then when you needed to say “that” before a “mae” phrase I’ve got muddled - I’m sure I heard “bo fi’n” too but I might be wrong - I tried re-listening but it goes too fast for me to grasp it. Could you please explain when to not use bo fi but bod and if possible how you complete the sentences involved. Including did I hear a bo fi’n mentioned? ( I’ve got M.E. so could you explain as basically as possible- I know I’m a pain!!!) 
I haven’t done the course, so I’m not familiar with the challenges, but I’ll have a listen and see if I can help. Is it level 1? And which version are you doing - north or south?
ah diolch! Yes level 1 and South - I think the bod / bo fi is also in challenge 5 - when I didn’t grasp it then I realised I needed to ask for help 
The way that I understand it is that one pattern is replaced by another. This is because the conjugated form of the verb-noun BOD changes.
So the formula dw i’n moyn is replaced by the new formula bo’ fi’n moyn Dw i —> bo’ fi
And the formula mae rhaid i fi is replaced by bod rhaid i fi. Mae —> Bod
This change in pattern occurs after certain verbs-nouns, such as to think or to believe. The change is something like using the English word THAT after those verbs:
I think THAT I… etc
To suss out which of those two changes one ought to make, we have to think back to the pattern we learned for the phrase that follows the word that.
For example, say we want to translate into Welsh: I think that I want to improve.
How did we learn to say I want? Dw i’n moyn. And that particular pattern, as noted above, is replaced by bo’ fi’n moyn in this context.
So we would say: Dw i’n meddwl bo’ fi’n moyn gwella.
And if we wanted to say: I think that I need to improve?
We learned Mae eisiau i fi for I need.
As per the pattern above, this would now become BOD eisiau i fi.
Dw i’n meddwl bod eisiau i fi wella.
So if you normally use Dw i / Dw i’n, change it to bo’ fi / bo’ fi’n. If you normally use Mae, change that word to bod.
Dw i’n siarad Cymraeg —> Dw i’n meddwl bo fi’n siarad Cymraeg.
Mae rhaid i fi siarad Cymraeg—> Dw i’n meddwl bod rhaid i fi siarad Cymraeg
ah wow - DIOLCH YN FAWR IAWN! That is really well put - and my M.E. brain understood it! (Seriously I got A levels back in the 80s but you’d never think that now- M.E. affects your concentration big-time! 
) I think that explanation might help others too- very well put! Thank you to Siaron for your help too 
Excellent explanation @LiamQ - I go offline at 7pm, so you beat me to replying! I’m glad that helps you @catherine-evans-3
diolch!