Adverbial clauses of condition state the condition (real or unreal) which is necessary for the realization of the action expressed in the principal clause. They may be introduced by the following conjunctions: if, unless, once, in case. There are also several conjunctions derived from verbal forms sometimes followed by the optional that: provided (that), providing (that), suppose (that), supposing (that), considering (that), given (that), granted (that), granting (that), admitting (that), presuming (that), seeing (that).
Conditional clauses introduced by if and other conjunctions (with the exception of unless) imply uncertainty. Therefore they often contain non-assertive forms of pronouns and pronominal adverbs, such as any, anybody, anything, anywhere.
If anything troubles you, you’d better tell me.
If anyone asks for me, tell him to wait.
Clauses beginning with unless express the only possible condition which will make the action in the main clause possible. Therefore they usually contain assertive forms like something, somebody.
Unless somebody interferes, there may be a disaster.
For the same reason unless-clauses hardly ever express unreal conditions. The exclusive meaning of unless accounts for the fact that, even if the condition is real, the unless-clause is not always equivalent to an if-not-clause. Thus the sentence: I won’t come unless you invite me (Я приду, только если вы пригласите меня.) and the sentence I won’t come if you don’t invite me –(Я не приду, если вы меня не приглашаете.) are quite different in their meaning.
The conjunction provided opens a clause containing some desirable condition for the fullfilment of the action expressed by the predicate in the main clause.
And you can do what you please, provided you do it neatly and don’t make a row over it.
The conjunctions suppose and supposing always imply that the condition is merely hypothetical.
I mean this: Suppose some other European pauper prince was anxious to marry Princess Anna and her fortune, wouldn’t that Prince have an interest in stopping this loan of yours to Prince Eugen?
Adverbial clauses of condition can be joined to the principal clause asyndetically. In this case we find inversion in the subordinate clause.
... should Frank marry to-morrow, I shall have no ground for blaming him.
Had she been an Englishwoman, Artois would have guessed her to be near fifty.
It would be a serious setback, were the talks to fail.
Instead of using a conditional clause containing the verb to be, it is possible to use a phrase consisting of the conjunction if followed by an adjective or a prepositional phrase. For example, instead of saying We will sell the car, if it is necessary, we can say We will sell the car, if necessary.
This unfortunate situation is to be avoided if possible.
If I were innocent, I'd rather be tried here; if guilty, in America.
If in doubt, ask at your local library.