There are different uses of verb to be. e.g. present and past continuous, passive voice, etc.
Present continuos: be + verb + ing. It is used for things happening now, at this moment. My brother is working in Germany. A: What are you doing? B: I´m sending a text message to Sarah. It is also used to talk about arrangements in the future. I´m not going to the match on Saturday.
Past continuous: was/were + verb + ing. It is used to describe an action in progress at a specific moment in the past. At six o´clock last night I was driving home. On Aprit 1st I was staying with some friends in the country.
Passive voice: You can say things in two different ways, in the active or in the passive. Picasso painted Guernica. (active) In this sentence, the focus is more on the person. (Picasso) Guernica was painted by Picasso. (passive) In this one, the focus is more on the painting. (Guernica) The passive can also be used when who does or did the action is not known or not important. My car was stolen last week. (I don´t know who stole it)
Make the present passive with am/is/are + past participle: These offices are cleaned every morning. Make the past passive with was/were + past participle: The pyramids were built by the Egyptians. Use by to say who did the action: The Lord of the Rings was written by Tolkien.