Complex sentences : complex sentence consists one principal clause and one or more than one subordinate clause. Complex sentence are made up with the help of subordinatiing conjunctions.
Eg.
1. He paid off the debts which his father had contracted.
2. Our publisher is a man who keeps his promise.
3. The reason why he succeded is known to all.
4. As soon as you have reached manhood, you will have to work for your living.
👆 Above all sentences contains one principal clause and one subordinate clause.
How to identify complex sentences
As, asif, as though, as that, as long as, as much as, as than, although, though, as far as, as soon as, according as, after, before, because, if, whether, even if, in case, that, so that, in order that, provided that, not withstanding that, than, now that, the moment, the minute, considering that, who, whom, whose, which, what, how, when, where, while, till, untill, unless, etc.
Note . If two are more than two sentences are joined with the help of above written word then these kinds of sentences called complex sentences.