The Irish sentence is in the present tense ("an gcasann tú?"), so the English translation must also be in the present tense. The past tense version of this sentence would be "Ar chas tú timpeall?" (which would then translate as "Did you turn around?")
Could I also use this if I'm asking someone to turn around? As in 'would you turn around'?
You could say An gcasfá timpeall? ("would you turn around?"), and it could be understand as a request, rather than a question, but you might be better off saying Ar mhiste leat casadh timpeall? - "Would you mind turning around?"
"Are you turning around?" is the translation of An bhfuil tú ag casadh timpeall?. The verb is Bí, not cas.
Thanks for responding! I'm not sure I entirely understand, though. (Which may be a function of trying to learn a language from scratch). At least to me, it looks like the original sentence/translation requires the addition of "to do" as an auxiliary verb to form the question. Generally, when translating into english "to be" is also acceptable as an auxiliary (though it does require a gerund). I'm sure you're correct, I just want to figure out why. So:
Are you saying that because "An gcasann tú timpeall?" uses the verb "cas" (which I read as "to turn") that "to do" is the only acceptable auxiliary to add? And is that because "An bhfuil tú ag casadh timpeall" uses the verb "Bí" in the form "bhfuil" to create the equivalent of a gerund phrase? Is the practice to default to "to do" as the auxiliary of choice if "Bí" is not present?
Generally, when translating into english "to be" is also acceptable as an auxiliary (though it does require a gerund).
That might be true if you are translating from a language that doesn't differentiate between the present simple and the present continuous. It is not true when translating between languages that have both. An gcasann tú timpeall? does not mean "Are you turning around?", and An bhfuil tú ag casadh timpeall? doesn't mean "Do you turn around?". A fluent Irish speaker or English speaker would never confuse those two sentences. The difference isn't subtle, it's almost as significant as the difference between the past tense and the future tense.
"Do you turn round" was marked wrong, I guess because it sounds more like a habitual thing, which would maybe translate "an bhfuil tú ag casadh timpeall" ?
If it was marked wrong, it was probably because whoever added the exercise didn't anticipate (or doesn't like) the ellision of "around" to "round". I don't see any difference in habituality between "around" and "round", and "are you turning around?" isn't habitual at all.