An adverbial clause of cause (reason) shows the cause of the action expressed in the principal clause. Adverbial clauses of cause are introduced by the conjunctions as, because, since, for, so, that, least, for fear (that), seeing (that), considering, in case, just in case, in view of the fact that, in so far as (insofar as), by reason of,on the ground that, for the reason that and some others.
The conjunction as is preferable when the subordinate clause precedes the principal.
As he was tired, he preferred to stay at home.
Since there is no help, let us try and bear it as best we can.
Mr Woods, I am here just in case anything out of the ordinary happens.
Censorship is feeble inasmuch as it does not protect anyone.
Adverbial clauses of purpose
Adverbial clauses of purpose state the purpose of the action expressed in the principal clause. They are introduced by the conjunctions that, in order that, so that, so as, so, for fear that, lest (чтобы не) and some others.
When a clause of purpose is introduced by the conjunctions that, so that, in order that, we find the analytical subjunctive with the mood auxiliary may (might) if the principal clause refers to the present or future; if the principal clause refers to the past, only the form might is used.
She opens (will open) the window that she may (might) get a breath of fresh air.
Не got up, cautiously, so that he might not wake the sleeping boy.
The conjunctions lest and for fear (that) introduce clauses stating what is to be prevented, as both the conjunctions have a negative meaning. Lest is now chiefly in formal or old-fashioned English and after this conjunction the analytical subjunctive with the auxiliary should is generally used.
She opens (opened, will open) the window lest it should be stuffy in the room.
“It’s a bit lighter in the park,” he said, “but take it (an electric torch) for fear you get off the path.”
The conjunctions in order and so as may also introduce non-finite infinitive clauses.
They were shoving each other out of the way in order to get to the front.
The best thing to do is to fix up a screen so as to let in the fresh air and keep out the flies.
It should be noted that the conjunctions that, so that, lest, so are not confined only to clauses of purpose: that may introduce subject clauses, predicative clauses, and object clauses; so that may introduce clauses of result, lest - clauses of cause, subject clauses, predicative clauses and object clauses; so - clauses of result and of cause.