You normally may not move out in the middle of your lease. You signed the lease so you are bound by it, just as the landlord is bound to the tenant for the term of the lease. Some exceptions do apply.
What if the landlord breached the lease?
If the landlord breached a express or implied promise in the lease in some significant way (e.g., by failing to keep the place habitable), you may have the right to terminate the agreement before it expires. This is called “constructive eviction.” That is, the landlord breached the lease by failing to fulfill a material condition of the lease such as keeping the premises habitable. Examples include, non-functioning pluming, loss of hot water, vermin infestation, mold, and any other major condition that renders your premises uninhabitable.
May the tenant find someone else to take her place?
If the lease has no language preventing you from assigning or subleasing, the tenant may try to find someone else to take her place. The new tenant will pay rent to the old tenant, and the new tenant will pay it to the landlord. If this new tenant fails to pay the tenant, the tenant still owes the rent to the landlord – unless the landlord accepted this new person as a replacement for the tenant. However, the landlord has no obligation to accept the new person as a replacement though the landlord may have to accept the new person as an “assignee” or “subtenant.” This might not be bad for the landlord as it might him two sources for the rent: the new person and – if that person fails to pay – the old tenant.
Sometimes your agreement will expressly allow the tenant to assign or sublease, but only with the landlord’s consent. If the tenant presents a potential assignee or subtenant to the landlord who is a good tenant, the landlord might be breaching the lease if he unreasonably withholds his consent. This might constitute a “constructive eviction” that allows the tenant to terminate the lease.
What if I simply move out?
If a tenant simply moves out during the lease, she has breached the lease, and the landlord may hold the tenent liable for the balance of the term. You might get sued. But in most states the landlord has an affirmative duty to try to “mitigate” the losses by renting the place as soon as possible to someone else. Even if the landlord is able to re-rent it quickly, you still might be liable for the landlords advertising costs and the like.