You might want to look here, to start:
Depending on your willingness to escalate, you should also contact an attorney. You may be able to recover rent.
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If you haven't already, you may also want to consider withholding rent due to the poor conditions in the apartment. This can often have the effect of giving a landlord new urgency to deal with problems. It can also encourage a landlord to start an eviction which may be a good thing in your case since at that point you could bring all of your counterclaims and defenses against him, including retaliation for bringing the eviction as a result of your lawful rent withholding. I agree with Erik that you should probably consult an attorney to determine exactly what defenses and counterclaims might be available to you as well as whether it might be a good idea to withhold rent in your particular situation, and if so, how best to go about doing so. Good luck!
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