when后面的时态是不是只能是一般过去时,而不能用过去进行时.eg：When the students are sing

When I came in,the students were singing.对.
when引导的时间状语从句一般用一般过去时或者一般现在时,while 从句一般用进行时.比如：
(1) He was still smiling when the door opened and his wife came in.
他正笑着的时候,门突然开了,他妻子走了进来.
(2) I was thinking of this when I heard my name called.
我正想着这件事时,突然听到有人叫我的名字.

第一句的时态是错误的，因为学生在唱歌已经是过去的事情
when后面并不一定是一般过去时，可以是when the students were singing, I came in.