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  1. Should these measures fail to restore order, harsher restrictions will have to be imposed.

Are “should” and “if” interchangeable at the beginning of a sentence? (Should he study hard, no doubt he would pass this course OR If he study hard, no doubt he would pass this course.)

If Not then in what cases we can interchange them.

  1. If he should study hard........OR Should he study hard.......

Is this sentence conditional?

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You can use should to express a condition, but should and if are not interchangeable.

With should, the verb is a bare infinitive. We make a clause conditional by inverting the auxiliary verb and the subject.

He should fail. - describing the expected outcome
Should he fail... - conditional.

With if, the verb is in present tense. When we make a conditional clause, no inversion takes place: we simply add the word if at the front.

He fails. - fact
If he fails... - conditional

Note that the use of should to form a conditional sounds very formal: it is not something that you would hear in everyday speech.

See section 258.3 of the Oxford Guide to English Grammar for more information.

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With 'if' the verb is not necessarily in the present tense. It might be considered a bit old-fashioned but you could say "If he were to fail..." I sometimes hear, even from British native speakers, and more commonly from Americans "If he would fail.." In the specific example quoted, to my ears there is a subtle difference between "If these measures fail..." and "Should these measures fail..." in that the former contemplates failure as a serious possibility, whereas in the latter it is a hypothetical situation that would call down even more repressive measures.

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