if 后面一定带一般现在时么?以及when的用法

1. if 引导两种从句,宾语从句和状语从句, 宾语从句时态没有限制的.我想你问的应该是状语从句,表示条件.
一般来说主将从现的情况较多.比如:
If I am free, I will go with you.
If I am free, I can go with you.
但是if的后面也可以跟现在进行时表示假设的一种正在发生的情况:
If you are crossing the road, please wait for the green lights.
总的来说,对于初学者,if引导的句子时态基本上是一般现在时,现在进行时,情态动词这几个.

当表示现在或者将来的时候,when的用法和if一样.
但是表示过去时,when后面的句子时态一般有两种:
一般过去时:When I got home, my mother was cooking.
过去进行时: When I was doing my homework, someone knocked on the door.

when he is old这种用法应该没有,一般都是表示过去when he was old...

if 后面不一定带一般时，如If I have studied harder, I would be accepted by Peking University now.如果我曾学得更用功的话，现在可能上北大了。
再如If I were you, I would take the chance.如果我是你的话，我会抓住这次机会。

when he is old 有这种说法，是表示将来“他老了的话”。如If he continues to waste time, he will be sad when he is old.