Landlord not repairing the maindoor of house. How to get him to do it?

In case your monthly rent is less that Rs.3500/- , you may send a legal notice under Section 44 of the Delhi Rent Control Act which provides remedy for every landlord to keep the premises in good and tenable conditions and to get the same repaired if needed, from time to time. On failure to do so, within a reasonable time, after sending the notice in writing, the tenant may himself / herself make the required repairs and deduct the expenses of such repairs from the rent or otherwise recover them from the landlord.

However, in case your monthly rent exceeds Rs.3500/-, subject to the terms and conditions of your rent agreement, you may issue a written letter / notice to the landlord to get the same repaired / replaced, failing which you can file a suit for mandatory injunction seeking directions from the court against the landlord to get the same repaired.