Security deposits
Can my landlord keep my deposit if I break the lease?
They can apply it to actual losses - unpaid rent while the unit sits empty, re-letting costs - but in most states they must try to re-rent, and they can't just pocket the deposit as a penalty.
Breaking a lease makes you liable for the landlord's real damages, and the deposit is the handiest source. But most states impose a duty to mitigate: the landlord must make reasonable efforts to re-rent, and your liability stops when a new tenant starts paying.
Ask in writing for an itemization of what the deposit covered - vacant days, advertising, actual costs. 'You broke the lease so you lose the deposit,' with no math, is not how it works in most states.
Do something about it
The Security deposit recovery turns this answer into action.
General educational information, not legal advice. Laws vary by state and city and change over time - for a specific situation, contact a local tenant-rights organization.