They are used to introduce subordinate clauses and join them to principal clauses/
main clauses. There are three kinds of subordinating conjunctions:

A) Simple subordinating conjunctions:
after, although/though, as, because, before, if, however, once, since, than, that, till/until, unless, when, where, while, whether

He came after I had left.

B) Compound Subordinating conjunctions:

I. as far as, as long as, as soon as, so long as, so far as, in as much as, just as

She garlanded him as soon as he arrived.

ii. in that, so that, in order that, such that,
now that, provided (that), considering (that),
granting (that), on condition that

You can go for the trek provided you are well.

iii. as if, as though, in case

Take extra money in case you need it.

C) Correlative subordinates

if … then
as … as
so …….as
just ….as
so ….the
such ….that
so ….that
no sooner ….than
hardly …. when
scarcely…. when
whether ….or
though ….yet

If I don’t return in an hour then call the police.

Note: Relative pronouns and Relative adverbs, too, can function as subordinate conjunctions:
Relative pronouns

who/whose/whom/which/what /that


Relative adverbs – when/where/why/whenever/wherever