2. The present subjunctive is sometimes used in the if-clause. This form commonly suggests more doubt than the present indicative.
If it rain to-morrow, I shall not go.
3. In a future condition which puts the supposed case rather vaguely, often with a considerable suggestion of doubt, a verb-phrase with should or would is used in both clauses.
If it should rain to-morrow, I should not go.
For the use of should or would in such clauses, see [§ 305].
A phrase with were to may replace the should-phrase in the if-clause. This form often emphasizes the suggestion of doubt.
If it were to rain to-morrow, I should not go.
The past subjunctive may stand in the if-clause instead of the should-phrase.
If it rained to-morrow, I should not go.
Note. The comparative amount of doubt implied in the different kinds of future conditions cannot be defined with precision; for it varies with the circumstances or the context, and often depends on emphasis or the tone of the voice. Thus, in “if it should rain to-morrow,” should may be so emphasized as to make the supposed case seem highly improbable, whereas an emphasis on to-morrow would have a very different effect. As to the subjunctive, its use is often due rather to the writer’s liking for that mood than to any special doubt in his mind.