Does a landlord have to make an effort to find a new tenant if a present tenant broke our lease or can he just do nothing have the townhouseleft empty?

Although you are liable for the rent for the balance of the term of the lease or until the place is re-rented prior to the end of your lease, the landlord cannot allow the place to remain vacant without making reasonable efforts to find another tenant.  The landlord is required to mitigate (minimize) damages (the amount the landlord is claiming you owe) by making reasonable efforts to find another tenant.  If the landlord does nothing and allows the rental to remain vacant, the landlord has failed to mitigate damages and the landlord's damages will be reduced accordingly.  Reasonable efforts on the part of the landlord to find another tenant will be determined by what other landlords in the area are doing to attract tenants; for example, posting a sign on the property advertising the vacancy, advertising the rental in the newspaper, in a local rental guide or online, etc.

Once the place is rented, your obligation to pay rent ends; however, if the landlord charges the new tenant less rent than you were paying, you may be liable for the difference in rent for the balance of the term of your lease.  The landlord has to have a valid reason for charging the new tenant less rent such as market conditions.  If the landlord does not have a valid reason for charging the new tenant less rent than you were paying, the landlord has failed to mitigate damages.  The landlord's damages will be reduced accordingly and you might not be liable for the disparity in rent.