Why can we not use the pattern "If S will V, S will V"?

In English grammar, the following pattern is regarded as a wrong pattern.

My parents will send me to a mental hospital if I will kill someone.

Solution 1:

The sentence "My parents will send me to a mental hospital if I will kill someone" is not grammatically wrong, but it sounds awkward and probably does not convey the meaning you have in mind. The meaning of "if I will kill someone" can vary with context:

• "if it is true that I will kill someone"
• "if I intend to kill someone"
• "if I agree to kill someone"

None of these options works particularly well for the sentence in question. Compare with the following sentences, which (to my ears at least) sound perfectly normal:

• "If you won't clean up your room, I won't let you have any dessert."
• "If you'll be good, I'll give you a present."
• "If the machine won't work, I won't waste my time trying to use it."
• "If you'll just listen to what I have to say, I think you'll change your mind."

But I take it you mean "My parents will send me to a mental hospital if I kill someone", which can be re-worded as "In the event that I kill someone, my parents will send me to a mental hospital". This is known as "first conditional". Not all languages do this the same way. In Latin, for example, this is called "future more vivid" and takes the form "future + future" instead of English's "present + future".