Your landlord can only deduct money from your deposit if you have broken terms of your tenancy agreement leaving the landlord worse off. Common reasons for deductions are:
The landlord can only charge to restore themselves to the position they would have been had you met your agreement. The normal decline in condition of the property and its contents (“fair wear and tear”) cannot be charged to the tenant, and must be factored in to the amount charged for repairs, redecoration, or replacements.
For example, if you have broken a five year old sofa you should be charged the value of the five year old sofa and not the value of a brand new one.