If defects occur during the rental period, report them immediately to the landlord.
Minor damage:
Defects such as scratches or stains on the wall, are generally not the landlord's responsibility, provided these defects do not compromise the use of the rental object.
Small repairs, on the other hand, such as tightening loose screws or cleaning the drain in the bathroom, must be dealt with by the tenant. The tenant is only responsible for repairs that do not require special expertise.
Normal wear and tear: As a rule, damage caused by wear and tear must be repaired and paid for by the landlord. As a tenant, you only have to pay to repair damage that you have caused, for example when you accidentally break something belonging to the apartment.  Send your bill for this type of damage to your personal liability insurance. The lifespan (web page available in German) of the damaged object is a determining factor in the calculation of who will pay for the damage. If damage was done to an old stove, its lifespan was likely to expire soon, and it would have been the landlord's responsibility to replace it. The landlord would, in this case, bear a large part of cost to repair the damage. If, on the other hand, the cooker you damaged is still almost new, you or your insurance company will pay the bulk of the repairs.
Defects:
If you notice defects that you as the tenant did not cause, you can demand that the landlord solve the problem and reduce the rent proportionately. You have an entitlement to a reduction in rent from the moment the landlord is made aware of the defect until it is remedied.
If, for example, the heating fails or there is a mould outbreak in the bathroom, the landlord must have the damage repaired by a professional and foot the bill.