One standard assumption is that the subject of a clause originates from the specifier of its predicate, following the predicate internal subject hypothesis. For example, the subject of a simple transitive sentence would originate from [Spec, VP] (or [Spec, vP]) and the subject of a copular sentence with an adjectival predicate would originate from [Spec, AP].
One question arises when DP predicates are involved. In a sentence such as "this is Mary's watch", the specifier of the DP is occupied by "Mary".
The subject is, therefore, unable to be base-generated in [Spec, DP]. Where is it base-generated then? How does the hypothesis account for this?