1 Their expertise is more extensive than { [what] I had expected }.
The clause "[what] I had expected" is a nominative clause acting as the object of the preposition "than". The subject is "I", the verb is "had expected" and the direct object is "what". The entire clause could be replaced by the pronoun "that": Their expertise is more extensive than that.
2 Looking in the mirror, she saw something move behind her.
As it stands, there is no subordinate clause. However, given only one additional letter, the following version does have one:
- Looking in the mirror, she saw { [that] something moved behind her }.
With "moved", the nominative clause acts as the direct object of the verb "saw". The subject is "something", the verb is "moved", and "behind her" is an adverbial prepositional phrase modifying the subject. The entire clause can be replaced by the noun "movement": Looking in the mirror, she saw movement.
With only "move", "something" is the direct object and "[to] move behind her" is the bare infinitive object complement.
3 I simply couldn’t believe { what the President was saying }.
The nominative clause acts as the direct object of the verb "could believe". The subject is "the President", the verb is "was saying" and the direct object is "what". The entire clause could be replaced by the pronoun "that": I simply couldn't believe it.
4 The man { who promised change } seems to be only a devious weasel.
The relative clause acts as an adverb modifying the simple subject "man". The entire clause can be removed without affecting the grammar of the remaining sentence: The man seemed to be only a devious weasel. The clause could also be replaced with the adjectival participle "lying": The lying man seemed to be only a devious weasel.
5 Recently, finding a job has become very difficult.
As it stands, there is no subordinate clause. "Recently" is an adverb, modifying the verb. "Finding a job" is a gerund phrase serving as the subject. "Has become" is the verb. "Very difficult" is an adverb phrase modifying the verb.
6 Robert promised me { that he'd send a postcard }.
The nominative clause acts as the direct object of the verb "promised". The subject is "he", the verb is "would send" and the direct object is "a postcard". The entire clause can be replaced by the pronoun "something": Robert promised me something.