The difference between “gonna be doing it” and “gonna do it”

If you say when something happens, you imply it definitely will happen, even if the precise timing is unknown. You use if when there's uncertainty about whether the event will happen at all.

It's quite common — especially in informal contexts — to use when/if as a shorthand way of signifying when, but allowing for the possibility that the event in question may not in fact occur.

Personally I tend to use the composite when/if rather than if/when, for no reason I can clearly explain. I can't easily search for those hyphenated forms to check others' usage, but I can use NGram to check the more extended equivalents. This covers well over a million instances, so I think it's statistically significant. But I have no idea why three out of four people put them the opposite way round to me...

I too have searched for a definitive answer to this question and not found one. My own way of differentiating grammar from usage errors for my English language learners is as follows:

• If the mistake contravenes a generalizable rule for all members of that word class, then it is a grammar mistake. Otherwise it is a usage mistake.

For example:

He live in Frankfurt contravenes the rule that verbs in the 3rd person singular present simple tense require an -s (with the exception of modals), and is hence a grammar mistake.

My grandfather is a very high man is a usage mistake. We can formulate a rule that high applies to mountains not people. But the rule applies to one member of the word class only and hence the mistake is one of usage.

On this basis, these errors are grammar errors:

• I play tennis yesterday.
• Do you have dog?
• I live in Frankfurt since 10 year. (3 errors)

And these are usage errors:

• I always enjoy to sleep late on Sundays.
• What is the reason of your lateness?
• She replied she didn't know the answer.

The issue is of more than purely theoretical importance because learners need to know whether they should consult a grammar book or a good dictionary/usage manual to find out if what they have written is correct.

It is interesting to note that two excellent resources for English language learners both have the word usage in their titles:

Garner's Modern American Usage and Swan's Practical English Usage.

Garner's book exclusively contains what I personally would define as usage issues, while Swan's includes numerous entries on what I would term grammar; including negation, passive, modals, determiners, etc. So it seems that even the experts can't agree on the meaning of the word.