What's a lease? A lease, simply enough, is a contract for the rental of property. As such, a lease is enforceable at law. If a tenant signs a lease (which is usually for a one-year term), the tenant is liable for rental payments during that period of time.
A few scenarios exist where a tenant can lawfully break a lease: active service in the military, health, safety or privacy violations by the landlord, or domestic violence abuse. Besides those scenarios, a tenant cannot simply walk away from a lease because they choose to. If they do, a landlord has a right to sue the tenant for the outstanding rent.
If a landlord is in a situation where the tenant breaks the lease, the landlord should try to lessen their damages by finding a new tenant. Even if it is possible for the landlord to sue the tenant for the full amount of owed rent, collecting a judgment against a tenant is difficult. The landlord is best off trying to recoup their losses by finding a new tenant as soon as possible. If the landlord does end up pursuing a case against a tenant, the landlord will have a stronger argument by showing that he or she made an effort to lessen the damages from the breach of the lease, rather than just sitting idle as the rent went unpaid. With this in mind, landlords should keep detailed records of all efforts to find a replacement tenant.
What's a tenant to do if they need to break a lease? Work out a deal with the landlord. For example, a tenant could offer to assist the landlord in finding a new replacement tenant, by helping to advertise and show the apartment. Both sides are best off trying to find an amicable solution to the problem in lieu of taking the matter to court.