Landlord and tenant law urban myths

Wikipedia describes an urban myth as a form of modern folklore consisting of apocryphal stories believed by their tellers to be true. Landlord and tenant law is a complex and often little understood area of law, and therefore there are quite a few urban myths.
The Landlord Law Urban Myths series is an attempt to debunk these myths and expose them for the misunderstandings and wishful thinking that they generally are.
The Urban Myths:
>> A short let cannot be an AST
>> You can’t have a proper tenancy without a written agreement
>> You can give you tenants probationary period before the tenancy starts
>> When a landlord lets a property, its still his
>> Tenants who stay on after the end of their fixed term are ‘squatters’
>> Tenancies must be ‘renewed’ when the fixed term ends
>> You shouldn’t accept rent when evicting a tenant
>> Landlords can take tenants property if they are in rent arrears
>> You can’t serve a section 21 notice during the fixed term
>> You can only serve a section 21 notice on tenants who are in arrears of rent
>> You have GOT to evict a tenant once the s21 notice expires
>> You don’t need to get a possession order if you use an abandonment notice
>> You can evict tenants really quickly using the accelerated procedure
>> Evicting squatters is difficult and takes a long time
>> A tenant who stays in a property a long time gets special rights