Pat (have) a lot of trouble with her parents this year. They want her ( be) at school every day and (expect) her (do) more at school. She (known) she (must) (work) hard (get) marks but she (can, not.) (arrive) at school on time and (be) ready with her tasks.

здесь вопрос про употребление инфинитива с частицей или без, а не про изменение формы....

Pat has a lot of trouble with her parents this year. They want her to be at school every day, and expect her to do more at school. She knows she must work hard to get marks, but she cannot arrive at school on time and be ready with her tasks.