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When he comes, I will leave here.

您的句子中come漏打了s, 第三人稱單數的現在式是comes.

記住這個原則:
表時間或條件的副詞子句, 要以現在式來代替未來式.
上面的句子就是一例.

另例:
If Michael gets scholarship, he will travel to America for schooling.
(表條件的副詞子句)

要注意: 只有副詞子句才適用.

The day will come when we will be able to travel in outer space.此句為何不能是 The day will com when we are able to travel in outer space.

這個句型看起來跟前面的句子很類似, 但這是不一樣的語意, when子句並非副詞子句,  而是依「尾重原則」後移的形容詞子句, 修飾The day. 原句是:
The day when we will be able to travel in outer space will come.
因為這個形容詞子句太長了, 恐影響聽者的理解, 可移到後面, 讓主要子句先講完, 這叫做「尾重原則」.
既非副詞子句, 就無須以現在式代替未來式了.

還有一句,I don't know when he will have finished his work.
為何用will have finished his work. 而不是finish this work. I don't know when he finish this work.

這一句中的when子句也不是副詞子句, 而是作為間接問句的名詞子句, 作為know的大受詞.
同理, 既非副詞子句, 就無須以現在式代替未來式了.