When I a. met 11. While I, a. looked 12. He David he was very tired. b. was meeting c. had met through the report I found a mistake. b. was looking c. had looked for a few hours through the forest before he reached the village. a. walked 13. she b. was walking a new dress for birthday party yesterday. c. had been walking a. Did buy b. Was buying c. Had bought 14. The weather changed my planes, as I to the picnic. a. went b. was going c. had gone

Решение:
1. В первом предложении «When I a. met 11. While I, a. looked 12. He David he was very tired.» правильный вариант — b. was meeting. Получается: «When I was meeting David, he was very tired.»
2. Во втором предложении «through the report I found a mistake.» правильный вариант — c. had looked. Получается: «I had looked through the report and found a mistake.»
3. В третьем предложении «for a few hours through the forest before he reached the village.» правильный вариант — a. walked. Получается: «He walked for a few hours through the forest before he reached the village.»
4. В четвертом предложении «a new dress for birthday party yesterday.» правильный вариант — c. had bought. Получается: «She had bought a new dress for the birthday party yesterday.»
5. В пятом предложении «The weather changed my planes, as I to the picnic.» правильный вариант — b. was going. Получается: «The weather changed my plans, as I was going to the picnic.»
Таким образом, правильные варианты ответов: 1b, 2c, 3a, 4c, 5b.