"Used to", is used to explain how things might be operated a certain way. So if I say that paint is used to cover walls, then that is the material that is taken and applied to that surface. The person may explain his use of paint by using the present tense and say I use paint to cover my walls. Then of course he is talking how he regularly takes the material and applies it to a vertical surface defining the rooms of a house. If the person then talks about the application as a generality, then he expresses it with "used to", otherwise he uses the simple present to say the same thing Actually, saying paint is used to cover walls is a passive form of saying I use paint to cover walls where in the first case the speaker wants the listener to know that the object paint, is more important than who uses it.
The term "used to", is also applied to things that are also abstract. So jokes can be used to get people to laugh. One can use the term in the past to talk about something is no longer in operation. I could say that I was used to telling jokes when I was younger. In fact the past of the term is often associated with a phrase in the past tense and the two are joined with interrogative words like when (when is used to ask questions).
Now used to, is often taught with supposed to. This term is used to talk about something, which has not happened and should occur. It also may be used to refer to someone. So if I say it is supposed to rain, then the weather is such that the person expects it to rain that day. If I say she is supposed to be here at nine, then I would be expecting her to arrive at that time. I can make the same statement in the past by substituting is with was and I have a statement which lets the reader know that at a previous time, the weather showed those same cloudy conditions. We are not in the habit of talking about how things are put into operation with supposed to alone. This where supposed to and used are hooked together to state something like, "Soap is supposed to be used to wash your clothes'. Then of course I am talking about the fact that in order to wash your clothes you need to use water.
Both "used to" and "supposed to" can be used in the plural form if you are referring to many things that put into operation or many things that should occur but don't. So water and soap are used to wash clothes or water and soap are supposed to be used to wash clothes.
