Learn English – Present tense for future events

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".

English has no future in the future because English has no future tense at all. English verbs approach tense from two perspectives: before now (past), now and after now (present/nonpast). As such, we can conjugate the verb to eat as follows:

I eat.

I ate.

But there is no way to conjugate the verb for the future, and so we resort to periphrastic constructions to form future aspects, which, for better or for worse, usually infuse other meaning into the sentence:

I will eat (volition).

I shall eat (obligation).

I may eat (possibility/permission).

These all imply future time (and thus form the future aspect), but may infuse undesired meaning into the sentence. Nevertheless, we also have less meaning-rich, albeit more verbose, ways of expressing future time:

I am going to eat.

I am about to eat.

So, although there is no future in the future tense, we can form a future in the future aspect by combining the foregoing constructions:

I will be going to eat.

I will be about to eat.

Both of which sound fine on occasion, but may grate on the ears (eyes) if heard (read) too often, especially in the passive voice: the food will be going to be eaten.

It is also worth noting that the present tense is often used for both present and future time, often making the future aspect seem too verbose where it is still grammatical. Consider the following pairs:

I am going home tomorrow. / I will be going home tomorrow.

He heads out in an hour. / He will head out in an hour.

In each pair, both sentences mean about the same thing and, at least where I live, the average Joe is more likely to say the first. This is merely something to consider, however, and it is not meant to discourage your idea at all.