must do or must to do??
I should use must do sth or must to do sth? or the both are all right?

Question

must do or must to do??
I should use must do sth or must to do sth?  or the both are all right?

Answer

"Must" is a modal verb never followed by "to". 
The same refers to the modal verbs  "can"; "should"; "could"; "may"; "might"

Answer

What have to do to start a conversation in english?

Answer

The modal verb "must" is always used without "to"