Write something down = to write something so that you don’t forget it. e.g. Please write down your email address.
Write in = to write a letter to an organization. e.g. The students wrote in with their questions.
Write something or somebody in = to add a scene or character to a book, play or film. e.g. They wrote in that part just to hire the superstar.
Write something off = to accept that money will never be re-paid. e.g. Our government is writing off the debt.
Write something off = when a damaged vehicle can’t be repaired. e.g. Following the accident the Honda has been written off.
Write out = to write information on a document such as a cheque or prescription. e.g. He wrote out a cheque for the money he owed us.
Write up = to complete something usually using notes. e.g. Finally she wrote up that report.