Here are the verbs in the correct forms:

1. I caught him (climb) **climbing** over my wall. I asked him (explain) **to explain** but he refused (say) **to say** anything, so in the end I had (let) **him go**.
2. When at last I succeeded in (convince) **convincing** him that I wanted (get) **to get** home quickly he put his foot on the accelerator and I felt the car (leap) **leaping** forward.
3. I'm not used to (drive) **driving** on the left.
4. When you see everyone else (do) **doing** it you'll find it quite easy (do) **to do** yourself.
5. It is pleasant (sit) **sitting** by the fire at night and (hear) **hearing** the wind (howl) **howling** outside.
6. There was no way of (get) **getting** out of the building except by (climb) **climbing** down a rope and Ann was too terrified (do) **to do** it.
7. We heard the engines (roar) **roaring** as the plane began (move) **to move** and we saw the people on the ground (wave) **waving** good-bye.
8. It's no good (write) **writing** to him; he never answers letters. The only thing (do) **to do** is (go) **to go** and (see) **see** him.
9. Why did you go and (round) **round** the field instead of (walk) **walking** across it?
10. I didn’t like (cross) **crossing** it because of the bull. I never see a bull without (think) **thinking** that it wants (chase) **to chase** me.
11. The people in the flat below seem (be) **to be** having a party. You can hear the champagne corks (thud) **thudding** against their ceiling.