- If the office is open until five, we’ll have plenty of time to go there this afternoon.
- If the office will be open until five, we’ll have plenty of time to go there this afternoon.
What’s the difference in meaning between the above two sentences?
The meaning is the same but the second variant of the sentence is grammatically incorrect. The correct one is: If the office is open until five, we’ll have plenty of time to go there this afternoon.
I’d say the sentence with ‘will’ in the IF clause in essence refers to a specifically stated future reality rather than to a condition. In other words, perhaps someone has just told you that the office will be open later than usual today – it will be open until five:
If (it is true that) the office will be open until five, we’ll have plenty of time to go there this afternoon.
I suppose you might say that the unspoken condition in that sentence is “it is true”.
This is not the most commonly used format for an IF sentence, but in my opinion it is one of several grammatically correct ways to use ‘will’ in an IF clause.
bbc.co.uk/worldservice/learn … v315.shtml
By the way, I’ve also heard that the use of future tense in IF clauses is old English usage, what do you think, Amy?
Commonly “will” is not used in if-clauses even when the statement is about a
future event. But sometimes the use of “will” in if clauses does not sound
 If you’ll help us, we can finish early. [Quirk, 1985]
 If drugs will cure him, this drug should do the job. [Quirk, 1985]
 If he will go to China next week, we should publish his book now.
 If you won’t arrive before six, I can’t meet you [Quirk, 1985]
 If it will make you happier, I’ll stop smoking. [Swan, 1980]
Thanks a lot, Stitifan, but as a non-native speaker, I really can’t make it out when the use of ‘will’ in IF clauses is not unnatural. Could you please be more specific?