1.i think i'll ride my bike.
2.if you do ,you'll be late.
3.i think i'm going to stay at home.
4.if you do,you'll be sorry.
5.how long have you been skating.
6.i've been skating for five hours.
7.i've been skating since nine o'clock.
8.how long did you skate?
9.i skated for two hours.