Поставьте глаголы в скобках в соответствующую форму, не did not know that the boss (to sign) the contract last week. i am not sure, if i (to leave) liberia next week. did he say that he (to write) a letter yesterday at five? we decided that we (to agree) to their proposals. i heard that you (to telephone) our partners in london next monday. we understood at once that the work (to be) a difficult one. when we ( to come) to the airport, the plane already (to land).

Не did not know that the boss had not signed the contract last week. I am not sure, if I leave/will leave Liberia next week. Did he say that he was writing a letter yesterday at five? We decided that we would agree to their proposals. I heard that you will telephone our partners in London next Monday. We understood at once that the work is/was/would be a difficult one. When we came to the airport, the plane had already landed.
"Поставьте глаголы в скобках в соответствующую форму" - не понятно, что имеют в виду под "соответствующей" формой. В некоторых случаях возможны несколько "соответствующих" форм.