how and when should we use the pharse "used to"?
"Used to" can have several different meanings.
It can refer to a previous situation: "I used to live in California, but I moved to Michigan in 1999."
It can mean getting acclimated or adapted to something: "At first I couldn't tolerate the humidity in Michigan, but now I'm getting used to it."
In the most literal sense, it describes a procedure: "The fork is used to hold the meat in place while you slice it with your knife."