前面讲的是 obligation，必须要做某事时，用 must, have (got) to, need to；

如果要讲某事不是 obligation，不是必要做的，可以用：

don't / doesn't have to 

needn't (need not)

don't / doesn't need to

不必……

我晚上不必工作。

I don't have to work in the evening.

I needn't work in the evening.

I don't need to work in the evening.

她晚上不必工作。

She doesn't have to work in the evening.

She needn't work in the evening.

She doesn't need to work in the evening.

旅行中你不必带食物。

You don't have to bring food on the trip.

You needn't bring food on the trip.

You don't need to bring food on the trip.

别担心。Don't worry (about it).

—> 你不必担心。

You needn't worry about it.

You don't have to worry about it.

You don't need to worry about it.

+ 我会处理好的。

take care of something 处理，负责

I'll take care of it.

—> 你不必担心。我会处理好的。

You needn't worry about it. I'll take care of it.

You don't have to worry about it. I'll take care of it.

You don't need to worry about it. I'll take care of it.