Bod is quite complex so don’t worry, it’ll take time to get comfortable with all its forms, but tp try and answer your question simply, “bo fi” is “that I”, whereas bod is the verbnoun. To use the examples you gave, Well I mi and rhaid I mi use bod because they would be followed by “be” e.g. rhaid I mi fod yn gynnar - I must be early.
An example with “bo fi” could be dwi’n gwbod bo fi’n gynnar - I know that I am early.
Thank you Siaron. It has helped a bit. How about using still? ie : ’ I think that I still must practice more’: would that be: ‘Dwi’n meddwl bod mi dal rhaid i mi ymarfer mwy’? Your help is much appreciated by the way!