τί σὺ ποιήσεις τὴν πόλιν ἑλών;

[jmorsay]

My translation: What did you do to the city?

My questions

1. Is this correct?

2. why is "σὺ" singular, when the subject in the verb pular? Does "σὺ" act as a neuter noun?

[billberg23]

Not correct, JM.  You need to take a good, long, hard look at ποιήσεις.  Does that ending -εις really look plural to you?  Does it look like a secondary (past) ending?  If the verb is past, why doesn't it have augment?  What's the function of that -σ- in the middle of the verb?

Then look at the root aorist active participle ἑλών.  Have you had the irregular verb αἱρέω in some recent grammar lesson? 

Final hint:  τὴν πόλιν is the direct object of ἑλών, not ποιήσεις.

[jmorsay]

New translation: what did you do to the captured city?

Is this correct?

thank you

[billberg23]

The point is that ποιήσεις is clearly second person singular future active indicative.  Its subject is modified by the (as I told you) aorist active participle ἑλών, whose direct object is τὴν πόλιν.