bad happen

  1. Don’t let anything bad happen.
  2. Don’t let anything bad to happen.
  3. Don’t let anything bad happened.
    Which is the right expression?

Only #1 is correct.

Could you please explain each sentence with its grammar?
We can say ‘bad thing’ but we can’t say ‘thing bad’.
How ‘anything bad’ is correct?

It is not the ‘bad thing’ part which is incorrect in your setences, It is the verbs.
In (2) ‘to happen’ is incorrect. It should just be ‘happen’ as in (1)
In (3) ‘happened’ is the wrong tense because you are speaking about the future.

Remember that you let something happen (no ‘to’) but allow something to happen.

Thanks Mr. Alan & Beeesneees.

ok .we got it .thanks .