Нужно употребить в [ ] нужный модальный глагол. Помогите пожалуйста)

15)Her questions are very easy, I [1]... answer them. 16) It [2]... be very difficult to learn such language, as Chinese. 17) I am very sorry, I [3]... not come yesterday. 18) [4]... they translate this text without using a dictionary? 19) Your children [5]... spend more time outdoors. 20) He [6]...work so hard now. 21) We [7]... not understand you at all.