如果是陈述句,那就在let前加don't,doesn't或者didn't.
They don't let me do it.
Our teacher doesn't let us smoke.(我们老师不让我们抽烟）----带有命令口气
注意：Our teacher lets us not smoke.（我们老师让我们不要抽烟）----带商量的口气
句子中使用的是let sb not do sth(让某人不要做某事）
如果是祈使句,分两种情况：
以Let's 打头,那就是在let's 后加not:Let's not watch TV now.
其他的这是let前加don't.
Don't let us waiting for long.
Don't let Jim go alone.