As a general rule, a tenant is bound to the length of the lease unless the landlord significantly breaks the law or violates its terms -- for example, by failing to make necessary repairs, or by failing to comply with an important lease clause. A few states have laws that allow tenants to break a lease because of health problems or a job relocation that requires a permanent move. Federal law and many similar state laws allow tenants who enter active military service and related government positions to terminate a lease early.
A tenant who breaks a lease without good cause will be responsible for the remaining rent due under the lease term. In most states, however, a landlord has a legal duty to use reasonable efforts to try to find a new tenant -- no matter what the tenant's reason for leaving -- rather than charge the tenant for the total remaining rent due under the lease.