<em>A sentence</em> refers to the pair of words which is complete in itself. In other words, a sentence contains a subject and a predicate or it may contain a main clause and one or more subordinate clauses to convey a command, exclamation, question or statement.
<em>Subject </em>refers to the doer of the action and <em>predicate</em> is the action performed.
Hence, when we notice in the above sentence, 'fit' is the verb which acts as a simple predicate and 'small mice' are the subject.
To determine the complete predicate, it answers,
The small mice fit where?- into coat pockets, as a result, the complete predicate is 'fit nicely into coat pockets.'
