But if you type 'The fish is swimming about you' it says incorrect answer though both under and about are given as options.
I translated this as 'the fish are swimming around you'. Is that not an acceptable translation?
Faoi gives more of a meaning of "about", like something is about something, so under really is the best one here.
If you want to translate 'around something' into Irish, you can use the adverb 'timpeall' followed by the genitive form of the noun. There are other ways of translating it, but I'm afraid that 'faoi' isn't one of them.
Why is "There is a fish swimming under you" incorrect? How would you say that?
'Tá iasc ag snámh fuibh' should do the trick.
'An t-iasc' means "the fish" because 'an' is the definite article, "the". Irish doesn't have the indefinite article "a", so if you want to say "a fish" you just say 'iasc'. E.g. 'caithim gúna' means "I wear a dress" rather than "I wear dress".
Why is "The fish swims under you" not accepted? Both "swimming" and "swims" are present-tense verbs.
"Swims" is the non-progressive form and the exercise wants the progressive form. Like English, Irish has a separate way to express the progressive form. "The fish swims under you" would be "snámhann an t-iasc fút/fúibh" while "the fish is swimming under you" is "tá an t-iasc ag snámh fút/fúibh"
Out of curiosity, in what situation would your even be in where there was a fish swimming under you other than in open water?
The preposition "ag" (at) is used with a verb in Irish to create the English equivalent of a "gerund"--the "-ing" form of any verb. You could think of it as being "at" doing something--"tá ag snámh" is "being at swimming" but the translation just uses the English gerund.