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A. Should he decides to cancel, I will inform you.
B. Should he decides to cancel, I would inform you.
C. Should he decides to cancel, there is a cancelation fee.
D. Should he decides to cancel, there was a cancelation fee.

1. Which of the above sentences are possible?
2. How are the possible sentences different in meaning?
3. Can we also say "Should he decided..."?

E. Had he decided to cancel, I will inform you.
F. Had he decided to cancel, I would inform you.
G. Had he decided to cancel, I would have informed you.
H. Had he decided to cancel, there is a cancelation fee.
I. Had he decided to cancel, there was a cancelation fee.

4. Which of the above sentences are possible?
5. How are the possible sentences different in meaning?
6. Can we also say "Has he decided..." or "Have you decided..."?
Comments  
A. should he decide (correct)

B. incorrect

C. should he decide (correct)

D. incorrect

All, if correct, would be decide.

E. incorrect

F. incorrect

G. correct

H. incorrect

I. Had he decided to cancel, there would have been...

For meaning, substitute "if" for "should"
Thank you for your answers, Philip.

'Has he decided to cancel, there is a cancellation fee' with 'has' in the beginning is never correct in conditional structure. 'Had' is the only one acceptable. Please confirm.
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Anonymouswith 'has' in the beginning is never correct in conditional structure. 'Had' is the only one acceptable.
Right.

CJ
You've basically got only two choices.

Should ... [base form] , will [base form] ....
Had ... [past participle], would have [past participle] ....

You might sometimes see a simple present tense instead of the will structure in the first one, though.

CJ
Thank you, CJ. That was really helpful.

If I've understood you correctly, 'should' normally takes the first or real conditional, and 'had', the third conditional. Please confirm.

As far as I know, most grammar textbooks only teach about 'if' conditionals and only a few include the alternate forms like 'should' and 'had'. I wouln't learn about this if not for this forum. Thank you, EnglishForward.com. Emotion: smile
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Anonymous'should' normally takes the first or real conditional, and 'had', the third conditional. Please confirm.
Yes. Exactly. I can confirm that. Emotion: smile

CJ
OK, thank you so much. You've been very helpful. Emotion: smile