13 was, had already set (считаю, что sit - это опечатка или ошибка)
14 did you hear, - I heard it (the news - ЕДИНСТВЕННОЕ ЧИСЛО!)
15 are you doing, am writing, did you receive, received
16 Has your son already done his lessons (еще одна ошибка)? - Yes, he has just finished... he is watching
17 Is it raining now? - No, it stopped raining... the sun is shining
18 has learnt, is translating
19 Has your child got... he got it
20 It's time to go. Have you finished your work?