The stars cannot be seen (by you) in the daytime.
2.It can't be done (by them) today.
3.The dictionary must be taken good care of (by you).
4.The bicycle must not be put (by you) here.
5.The exercises can be done (by you) tomorrow.
6.She must be sent to a hospital (by us) at once.
7.Can it be finished (by you) in two hours?
其实这些句子里的by sb都是可以省略的，平时真正说话也是省略的，既然你这里没有明确说明可不可以把人省略我就都翻上去了，但是其实是省略来的地道的多。看你们老师的要求了。
