A fixed-term lease means your tenancy automatically ends when the lease term is up. Until then, you're on the hook for the rent and other payments for the whole duration of the lease. You can leave early if the lease says you can or if you negotiate an early release with the landlord. If you just take off without paying rent, you could face serious consequences.
TL;DR (Too Long; Didn't Read)
If you leave early without permission, the landlord can sue you for rent for the duration of the lease, even for the months you weren't there, unless he's able to rent the space to someone else.
Early Termination Clause? There May be No Penalty
An early termination or early-release clause gives you the right to terminate the lease early in certain circumstances. Some clauses give you an out after a certain period, for example, six months into the lease term. Others let you terminate the lease if you lose your job or encounter other hardship circumstances. Usually, you'll have to give notice of your intention to vacate a month or two in advance. Follow the notice requirement closely or you might miss your opportunity to leave.
No Early Termination? The Landlord May Sue
Signing a lease means that you agree to pay the rent and observe the lease obligations right up until the end of the lease. If you leave the apartment early without the landlord's permission, then the landlord is entitled to collect rent from you for the duration of the lease, even for the months you weren't there. If you do not pay up, the landlord can sue you to recover what he is owed. For example, if you sign a 12-month lease and agree to pay $1,000 per month, but you terminate the lease early and leave after only eight months, the landlord can sue you for the remaining four months of the lease, even if you don't live there anymore, unless he finds someone else to take your place. In that case, he can only seek the rent for the period during which the property was vacant.
Landlord Must Try to Re-Rent
While the landlord can file a claim for back rent, in many states, she has a "duty to mitigate damages." This means that she must minimize your financial obligation and help herself reduce her own damages by attempting to re-let the apartment. Most judges will allow the landlord at least a month to find a new tenant, so at a minimum, you can expect to forfeit a month's rent. You'll also be responsible for the landlord's costs of advertising and showing the apartment, and other out-of-pocket expenses the landlord incurs as result of your actions.
Negotiating Your Way Out
If you don't have an early termination clause and you can't afford to pay the rent left on your agreement, then you could negotiate a way out. If you're lucky, your landlord will let you go. You can increase your chances by making sure the landlord is not out of pocket, for example, by agreeing to cover the rent for a few weeks until the landlord finds a new tenant. You could also offer to up your security deposit to help the landlord maintain the apartment until a new tenant moves in.
When Leaving is Justified
Landlords have a legal duty to keep the apartment in a safe and habitable condition. If the landlord repeatedly ignores your request to fix faulty plumbing or get rid of mold, then you can break the lease without penalty. Most states have specific rules you must follow if you're leaving because of a major repair problem, so do take legal advice before you move out.