"Where is the clinic?"
Translation:Πού είναι η κλινική;
There are a handful of verbs that don't trigger accusative, είμαι and υπάρχω being the two most common.
In more technical terms, this is because we only use accusative for the objects of verbs, and when we say something is something (I am a man, είμαι άντρας), or that something exists (there is a man on the road, υπάρχει ένας άντρας στο δρόμο), άντρας is not actually being "actioned" by the verb in either sentence. The verb in these sentences is a bit like an equals sign in maths, and accusative is not used with it.
Anyway, it's easier at this stage, I think, just to remember not to use accusative for nouns modified by είμαι and υπάρχω.
Thanks for the explanation that has made things clearer for me. Are there any other verbs that follow this rule
γίνομαι and φαίνομαι. There are quite a few more, I just can't recall them off the top of my head :-)
Further to spdl's explanation, it may be useful to consider the example as a statement: "The clinic is... (there)." This shows what the subject of the verb is, and that should be in the nominative. You can also replace "the clinic" with the third person pronoun: "Where is (s)he?" / "(S)he is there". For the basics of nominative/genitive/accusative you're still going to get he/his/him in Greek. ;)