A complex sentence has at least one main clause and one or more subordinate clauses. The subordinate clause may be a noun clause, an adjective clause or an adverb clause.

Be able to

Be able to has similar meaning to can and could.

Getting something done by somebody else is expressed by the structure have + object + past participle.

The infinitive is often used after be (is,am, are, was and were) to talk about an agreement or an arrangement. It may also be used to give an order or to express a complaint.

The present perfect tense generally represents an action which has been completed within a period of time that extends up to and includes the present time.

Subject + transitive verb (present perfect) + object + adverbials or complements

Subject + verb + (conjunctive + infinitive)

A conjunctive is an interrogative word which also serves as a conjunction. Examples are: where, what, when and how.

Must

The auxiliary verb must can be used to express probability or inference.

She must be about fifty years old.
You must have been mad to do such a thing.
She must have received your letter.