"The house is massive, I suppose."
Translation:Mae'r tŷ yn enfawr, am wn i.
Here "am wn i" is translated as "I suppose. But "I presume" means exactly the same.
'I presume' perhaps has more of a sense of confidence than 'I suppose'. Welsh has a different word for 'presuming', rhagdybio, which is closer to 'presupposing' than 'supposing'.
By and large, we stick with the most usual meanings in the course translations. This course is only an introductory one, and knowledge of other meanings In various contexts will come from a lot of conversation, reading and writing as you study further.