“do nothing but do”中后面一个“do”有没有时态变化？

没有时态变化。

1、当but前是实义动词do的各种形式时，but后接不带to的动词不定式，即接动词原形。如：

Mary did nothing but write a letter last night.玛丽昨晚只写了一封信。

We can do nothing but ask you for help.我们只得请你帮忙了。

We hate doing nothing but look on.我们讨厌袖手旁观，什么事也不干。

2、but 前有can/could, can't/couldn't(help),can't choose等时，but后接不带to的不定式。如：

I could not help but realize that something was wrong.我这才真正意识到出了什么事了。

One's world outlook can't but come through in what one says and does.一个人的世界观必然会在他的言行中表现出来。

The young woman could not choose but leave her children at home.那年轻妇女只好将孩子留在家中。

3、but 前有非谓语动词to do时，but后接的不定式可以带to，也可以不带to。如：

There was nothing for me to do but (to) wait for him.我只得等他了。

So there was nothing to do but (to) follow.没办法，只好跟着。

Now I have nothing to do but (to) go back home.现在我无事可做，只有回家。