grammaticality - Future tense in conditional clauses

All the textbooks I have ever come across during the course of my studying English emphasize that future tense should not be used in conditional clauses.

For example,

If it rains in the evening, we will not go for a walk. (if it will rain in the evening...)

We decided to go for a walk if it didn't rain in the evening. (...if it wouldn't rain in the evening)

However, in the following sentence I'm really inclined to use the future tense.

Don't implement this feature if it will significantly increase the complexity of the user interface.

According to all the rules I know of, the future tense is illegal here. However, my gut feeling tells me that the sentence is correct. If I am wrong, the question ends here. Otherwise please read on. I find the last example different from my first two because:

• In the first examples we must wait and see if the condition is true, and then make a decision accordingly, whereas in the last example, we must actually analyze/predict/forecast the future in order to make the decision in the present.


• _{(might be irrelevant)} In my first language - Armenian - where we have a special mood for conditions, the translation of the third sentence actually uses indicative, whereas the first two use that special mood (the conditional mood, as it were).

Since the second would-be principle is easier for me to experiment with, I noticed that every time a condition uses the indicative mood in my language, I'm inclined to use the future tense in English. As another example:

I will give you the money if it will make you happier.

Am I imagining things or are my examples of the future tense in the conditional clause valid? If they are valid, what rule would you suggest to distinguish the cases when it's OK? (I do realize that translating a sentence to another language and analyzing the translation doesn't really count as a rule).

Answer

I think the difference between the two types of examples that you've exhibited is the relative placement in time of the action in the "if" clause, and the action in the other clause.

• If it rains in the evening, we won't go for a walk - here, the event of raining occurs BEFORE the decision about whether to go for a walk.


• My teeth will rot if I eat too much sugar - presumably, I'll be eating the sugar BEFORE my teeth rot.


• If it will significantly increase complexity, don't implement this feature - here, the increasing of complexity occurs AFTER the implementation of the feature.


• I will give you money if it will make you happier - here, you becoming happier occurs AFTER I give you the money.

In all the cases where the "if" part happens first chronologically, we use the present tense. In the cases where the "if" part happens second, we use the future tense. However, because sentences of the first type are far more common than sentences in the second type, a good rule for learners to adopt is "don't use the future tense with IF".