In matters of real estate lease transactions, landlords have several duties:
Upon signing a lease, the landlord has a duty to actually deliver possession of the property, and take any actions necessary to facilitate such delivery. This means granting access to the property and providing keys or garage fob, for instance.
Landlords have a duty to maintain the property in a reasonable state of repair at no expense to tenant unless, as specified in the lease agreement, the tenant is responsible for the damage. This duty to repair does not extend to so-called "wear and tear" of the property, but rather the physical failure or damage of property functions such as a leaking roof; a failed hot water heater; non-functioning stove.
Quiet use and enjoyment (for residential properties only)
Landlords have a duty to ensure a tenant's quiet use and enjoyment of the property. This means that if the landlord causes substantial interference with the tenant's use and enjoyment of the property so severe that the tenant ends up vacating the property, the tenant's obligations to pay rent are excused and additional remedies, like damages, may be available. This is known as constructive eviction.
Warranty of habitability
Applying only to residential properties, this is an implied promise that requires the landlord to provide property that is reasonably suited to residential use, i.e., property that is fit for human habitation. Failure to satisfy this duty can trigger a constructive eviction claim.