You must do it yourself.

你必须自己做。

You must do your homework.

你必须做作业。

You must do well on this task.

你必须做好这项任务。

If you get a chance to learn to dance, you must do this because it is fun.

如果你有机会学习舞蹈，你必须这样做，因为它很有趣。

But you must do it.

但你必须那样做。

You must do something, you must take action.

你必须要做点什么，你必须要行动起来。

What you must do, do cheerfully.

必须要做的事，开开心心去做吧。

You must do the things you think you cannot do.

你必须要去做你认为自己无法做到的事。

Since you promised, then you must do it.

既然承诺了， 就一定要做到。

Why? You must do something wrong.

为什么？你一定做错了什么事。