"Are you swimming?"
Translation:Ben jij aan het zwemmen?
Please read previous comments. Other users have already answered that.
So is the verb always in the form it would be if you were speaking about multiple people (i.e., zwemmen v. zwemt for this example)?
I think that the word is in the infinitive verb: zwemmen, slapen, lopen etc.
Yes, we're using the infinitive form here just like when it is used together with plural subjects (like "jullie zwemmen").
I think it's because we're using "zijn (ben) + aan het" here that the infinitive form is used.
Het zwemmen is technically a verbal noun here. This is similar to English, because only one verb can be conjugated in a given clause. The only difference is that English uses a present participle for the second verb in the progressive, whereas Dutch uses a verbal noun (which is the same form as the infinitive), but in both cases those are non-conjugated forms.
Can someone please explain why it is 'ben jij aan het zwemmen', and not 'bent jij aan het zwemmen' when it's 'jij bent' in present tense?
It is an exceptional case (irregularity) of "ben" + "je/jij".
When "ben" goes first it would be without "t", as in "ben je/jij ...".
Otherwise it'd be with "t" as in "je/jij bent ...".