i don't know which one to choose.请造几个类似的句子

I don't know why i am not happy.
I don't know where is the nearest post office.
I don't know how to study english.
I don't know what has happened.

I don't know which one is correct

i don't know which one to choose.请造几个类似的句子

I don't know how to do it.

I can't remember what to do next.

I want to know when to start.