В приведённом ниже тексте на место пропуска (3) нужно добавить глагол: Tom Smith was a writer. He 1) ... (to write) detective stories for magazines. One evening he 2) ... (to be not able) to invent an end for a story. He 3) ... (to sit) with his typewriter in front of him, but he 4) ... (to have) no ideas. Then he 5) ... (to go) to the cinema. When he 6) ... (to leave), a burglar 7) ... (to break) into his flat and 8) ... (to read) his story. The visitor 9) ... (to leave) Tom a note: “I have read your story and I 10) ... (not to think) much of it. I 11) ... (not to go) to steal anything tonight, but I will return, when you become a successful writer.” Tom 12) ... (to study) the burglar’s suggestions and 13) ... (to write) the rest of the story. He 14) ... (to be) still not a successful writer, and he 15) ... (wait) for his burglar to return. Before he 16) ... (go out) in the evening, he always 17) ... (to leave) a half-finished story near his typewriter. Варианты: sat; were sitting; was sitting.

В предложении описывается действие, происходившее в определённый момент в прошлом: Том сидел перед печатной машинкой, пока у него не было идей. Поэтому используется Past Continuous: was sitting.

Ответ: was sitting.