6 must
7 must
8 mustn't
9 mustn't
10 mustn't
11 We always go to Moscow in summer
12 We usually stay in a hotel
13 Our grandparents never come with us
14 We often go to a cafe in the evening (we одно лишнее)
15 Am
16 does
17 do
18 is
19 do
20 are
21 does