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Hi, everyone. I have a question about tense which I couldn't figure it out. Almost all my grammar book says we will use "present tense" to express "future events" in if-clauses.

If it rains tomorrow, I'll stay at home.
If it is fine tomorrow, we will go swimming.

The weather condition is something happens in future, but we use present tense in if-clauses. But I saw some similar sentences use future tense in if-clauses:

If the office will be open until five o'clock, then we'll have plenty of time to go there this afternoon.
If it will help, I'll lend you some money.
I'll give you 100 dollars if it will help you go on holiday.

Could you enlighten me with what's going on here? Why sometimes we use present tense in in if-clauses and sometimes don't? And if it's convenient for you, could you also tell me if I can use "will" in the first 2 sentences? Like:

If it will rain tomorrow, I'll stay at home.
If it will be fine tomorrow, we will go swimming.

Please help me, thank you in advance.
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ViceidolIf it will help, I'll lend you some money.
I'll give you 100 dollars if it will help you go on holiday.
This often happens with the verb help. In my opinion, it's an abbreviated form of If you think that it will help, which is an if-clause in the present tense. (think is in the present tense.) The it will help part is therefore actually an implied subordinate clause.
The same doesn't work for most other verbs. You have to say "you think" explicitly in those cases. So either of these is possible:
If it rains tomorrow, I'll stay at home.
If you think that it will rain tomorrow, I'll stay at home.
Note, however, that while the person you speak to is in a better position than you to know if something will help him, he is not in any better position than you to know whether it is going to rain the next day. So that's why it's reasonable to say you'll do something if it rains, but less reasonable to do so just because someone thinks it will rain. And conversely, it's reasonable to say you'll do something if it helps OR if someone thinks it will help.
CJ
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To me, in this context, "will" has a sense of purposeful and directed activity, as opposed to something that "just happens". So "If it will rain tomorrow..." is almost always wrong because raining is something that "just happens". On the other hand, "If it will help/prevent/affect you..." are often OK because the action that's being proposed has direct purpose with respect to the other person.

"If the office will be open until five o'clock..." is, to me, usually wrong because the intended meaning is usually "I don't know if it will be open, and whether it is open or not is just something that may or may not happen, irrespective of my desires/wishes" (unlike CJ, I get no sense that "if" here means "since/because"). But "If the office will stay open for us..." is reasonable because it implies that they may make a special case to stay open.
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Comments  
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Native speakers tend to use present for future in these sorts of sentences. Where you see the future used, the speaker is thinking a bit more strongly of the future situation, I think-- the fact that it certainly will not happen now. I suppose also that the simple-present-as-future guideline enters in: an acknowledged future necessity (as in 'The train arrives at 2 pm tomorrow').
 CalifJim's reply was promoted to an answer.
ViceidolIf the office will be open until five o'clock, then we'll have plenty of time to go there this afternoon.
This case is different from the one with help.
To my ear, these two clauses are different in meaning:
1. If the office is open until 5
2. If the office will be open until 5
Clause 1 is the true conditional. We don't know whether the office will be open until 5, but if it is, then there will be some consequence.
Clause 2 is not a true conditional. We know that the office will be open until 5 in this case, and because it is, some possibility exists. It's like saying:
[Since / Because] the office will be open until 5 (as we know it is), we'll have plenty of time....
At least this is how the two versions strike me.
CJ
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 Mr Wordy's reply was promoted to an answer.
you need to use:

If it rains tomorrow, i'll stay at home.

If it is fine tomorrow, we will go swimming