Is the following clause correct?
I found it in a grammar book for French students:
If I should come across Mike, I would....
A native assured me 'should' can not be used in a conditional clause .
1. If X should happen, do Y.
2. If X should happen, will you do Y?
3. If X should happen, I'll do X.
4. Should X happen, do Y.
5. Should X happen, will you do Y?
6. Should X happen, he'll do Y.
I would agree with Harry; in British English, the use of "should" in conditional sentences, as exemplified above, is moderately common.
It occurs both in formal contexts, such as medical instructions, and in informal contexts, such as football commentaries, e.g.
7. If the symptoms should persist, see a doctor immediately.
8. Should he score now, what a debut that would be!
It may be less common in other kinds of English, though (e.g. American English).
All the best,
This use of 'should' is much more unusual in AmE, and when it is used, it tends to sound very formal and/or old-fashioned.