(副詞子句)Why the present tense in the adverb clause for a future condition?

It is a question raised by a student in Class 216 while a teacher was teaching the concept of the adverb clause. Is there any reason for that? To me, the student was clear-minded and eager to know the reasons for each grammar rule. It is no wonder that he majors in science in high school which requires critical and logical thinking.

為什麼副詞子句中，使用現在式代表未來式？
解釋問題之前，先來看個例句。
e.g. If it rains tomorrow, I will stay at home.
同學或許會問，為何有tomorrow不是用will V未來式？因為英文時態扮演重要角色，清楚點出事情發生先後順序，這跟中文沒有清楚時態有別。挑定某一時態表示用者清楚知道事件發生時間，從這副詞子句我們可以推敲它在表示某一條件，事情發生與否仍然是未知數，時間不明也非用者表達重點，就用基本的現在式，表示設下的條件等...
"條件"
1. 現在簡單式代替未來簡單式
e.g. When he comes, we will see him.
2. 現在完成式代替未來完成式
e.g. He will leave here after he has finished the project.
vs. "時間" (清楚知道事件發生與否，不知你是否理出差異呢？)
e.g. Tell me when he will come.
e.g. Do you know when the train will arrive?

不知是否解答你的問題，若還有疑問歡迎再提出，一起解開英文神奇秘辛XD