Learn English – Please don’t forget to ring me when you get home

Why is this correct?

You ask why the first example is "correct", and why the second is "incorrect", and there are several things to consider in the question.

In maths, we can say with certainty that 1 + 1 = 2 is "correct", and that 1 + 1 = 42 is "incorrect". Language, though, is not governed by as strict a body of rules as maths! "Correct" and "incorrect" in the context of English usage may not be appropriate terms. In any language, "correct" usage is that which is readily understood and employed by native speakers of that language. An instructor may tell you that something is "correct", and you should probably agree—as long as you understand that it is wiser to think of a given usage as being natural or best than as being correct.

The "why" is an entirely different question. Why do we (as in this case) use a present tense to describe actions that clearly take place in the future? Linguists have a variety of theories about the reasons for this and the other oddities in English and other languages; but for our purposes, it may be best simply to say:

"Why? Because that's how we say it in English!"

The Grammar

The remainder of this answer is concerned with grammar, and here we are on firmer ground. Your sentence contains a subordinate clause introduced by the subordinating conjunction when.

Please don't forget to ring me^{MAIN CLAUSE}

when^{SUBORDINATING CONJUNCTION} you get home.^{SUBORDINATE CLAUSE}

The most common subordinating conjunctions of time in English are when, after, before, until, since, while, once, as and as soon as. In clauses with these conjunctions, we often talk about the future by using the present tense. This is what your example demonstrates:

• Please don't forget to ring me when you get home.
      NOT_{Please don't forget to ring me when you will get home.}
  
• They want to be ready when their car arrives.
      NOT_{They want be ready when their car will arrive.}
  
• She will come home when she is ready.
      NOT_{She will come home when she will be ready.}
• Dave will make dinner as soon as Mary comes home.
      NOT_{Dave will make dinner as soon as Mary will come home.}
• Don't answer the phone until you find a pencil.
      NOT_{Don't answer the phone until you will find a pencil.}
• I am leaving after JayZ performs.
      NOT_{I am leaving after JayZ will perform.}

Swan, in Practical English Usage (p 573) tells us:

2 present instead of future: I'll write when I have time

Present tenses are often used instead of will + infinitive to refer to the future in subordinate clauses. This happens not only after conjunctions of time like when, until, after, before, as soon as, but in most other subordinate clauses...

This can happen even if the main verb is not future in form, provided it refers to the future.     Phone me when you arrive
    Make sure you come back soon

The British Council article on "verbs in time clauses and if clauses" includes more examples and explanations.