If the lava will come down as far as this, we will evacuate these houses

Question

Someone said the following sentence is grammatically correct.

If the lava will come down as far as this, we will evacuate these houses.

I think this is grammatically incorrect because there is the following grammar rule. http://www.englishcafe.jp/englishcollege/etense2/e1-2-4.html "In the time and the conditional clauses, both of the present tense and the past tense are used, but "will" and "shall" is usually not used even if it means the simple future. It is inevitable that the present tense is used instead."

I wonder which one is correct.

EDIT Some people say that there are exceptions that we use "will" in if-clauses.

Practical English Usage §260 If … will

We normally use a present tense with if (and most other conjunctions) to refer to the future. I’ll phone you if I have time. (NOT … if I will have time.) But in certain situations we use if … will.

1 results We use will with if to talk about what will happen because of possible future actions – to mean ‘if this will be the later result’. Compare: – I’ll give you ￡100 if I win the lottery. (Winning the lottery is a condition – it must happen first.) I’ll give you ￡100 if it’ll help you to go on holiday. (The holiday is a result – it follows the gift of money.) – We’ll go home now if you get the car. (condition) We’ll go home now if it will make you feel better. (result)

2 ‘If it is true now that …’ We use will with if when we are saying ‘if it is true now that …’ or ‘if we know now that …’. If Ann won’t be here on Thursday, we’d better cancel the meeting. If prices will really come down in a few months, I’m not going to buy one now.

3 indirect questions: I don’t know if … We can use will after if in indirect questions. I don’t know if I’ll be ready in time. (NOT … if I’m ready in time.)

4 polite requests We can use if + will in polite requests. In this case, will is not a future auxiliary; it means ‘are willing to’. If you will come this way, I’ll show you your room. If your mother will fill in this form, I’ll prepare her ticket. Would can be used to make a request even more polite. If you would come this way …

5 insistence Stressed will can be used after if to suggest insistence. If you WILL eat so much, it’s not surprising you feel ill.

Edit 2(19 July 2015) Some person wrote to me as follows.

(a) If the lava comes down as far as this, we will evacuate these houses.

(b) If the lava will come down as far as this, we will evacuate these houses.

Syntactically, both (a) and (b) are correct. But semantically, (a) is not understandable because if the lava ( from the volcano ) comes down as far as THIS, it will be too late to evacuate THESE houses. So if the lava WILL ( = is likely to ) come down as far as this, we must evacuate these houses immediately.

Edit 3(20 July 2015) I posted a similar question here(https://english.stackexchange.com/questions/260903/if-the-lava-comes-down-as-far-as-this-we-will-evacuate-these-houses).

Answer 1

You are absolutely correct. You should not use "will" in a conditional like this. Instead, you should use "does".

If the lava does come down as far as this, we will evacuate these houses.

Or "comes down"

If the lava comes down as far as this, we will evacuate these houses.

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Update July 18

My First answer doesn't go into enough detail. I realized why there is some confusion about whether the sentence is correct or not. There are two possible situations where you would want to say this sentence.

Scenario 1

There is a point (Let's say 5 miles from the houses) and if the lava reaches this point, then the houses must be evacuated. If the lava stays 6 miles, people can stay, but as soon as it hits the 5-mile point, everyone must evacuate.

Scenario 2

There is a point (Let's say right next to the houses) that if we can predict that the lava will reach this point people must evacuate. As soon as we know that the lava will eventually reach this point, the people will be evacuated.

Part of this confusion, is that I assumed that you meant scenario 1, and never thought about scenario 2. If you mean to say scenario 1, then it is incorrect to use "will". You are talking about when it does reach this point, not when it will reach this point. So you should say

If the lava does come down as far as this...

or

If the lava comes down as far as this...

However, if you want to say scenario 2, you should use "will". So really it is correct depending on your meaning. Although it is more clear and easier to understand if you instead say something like:

If the lava is going to come down as far as this...

Answer 2

The following sentence does not parse well for two reasons.

1. As previously stated, you should not use will in a conditional statement.
2. as far as this uses two comparison statements in quick succession.

If the lava will come down as far as this, we will evacuate these houses.

The following sentence is easier to parse.

If the lava comes down this far, we will evacuate these houses.

Answer 3

There is always confusion in this matter that plagues people who are trying to learn English as a second language because, in older English, perhaps only 100 years ago or so in the United States, this type of "if" clause took the present subjunctive conjugation, so that's the reason that it doesn't correlate correctly.

"If the lava come down as far as this, we will evacuate these houses." (using present subjunctive)

"But if the courses be departed from, the ends will change." (Ebenezer Scrooge in Charles Dickens' A Christmas Carol, published December 19, 1843)

I hope this might make sense of why "will" is not used in the protasis of "if/then" clauses that contain future possibility. We see and hear people trying to preserve the present subjunctive in these clauses all the time. For example:

"If the lava should come down as far as this, we will evacuate these houses."

"Should the lave come down as far as this, we will evacuate these houses."

"In the event that the lava come down as far as this, we will evacuate these houses."

"In the event that the lava should come down as far as this, we will evacuate these houses."