Question

СРОЧНО! Нужно найти ошибки:
1. Martin was advised to have been at the airport an hour and a half before the departure in case some changes would take place.
2. As you walk through Oxford you seem to be living in history, so many things called up events and figures of the past.
3. We would have spend much more time in this wonderful city if we had known how many places of interest it had had.
4. There were some boys and girls in the water but they might not swim very well as I realized after I started to watch them.
5. In the medieval library of Merton College you can see all the chained books and the old benches just as they had been in the 13th century.
6. He had been listening to other speakers with great interest since the time the conference began, taking a special interest in figures presented.
7. School studies schould help young people to find their places in society after having left school.
8. Hardly had he entered Harvard University, when he started to realize that he chose a wrong specialty.
9. The children could be more attentive while reading the rules, it's no wonder they have made so many mistakes.
10. The damage caused by air pollutants has already noted and shows to be extensive.
11. He is sure that she wouldn't have done it yesterday if she had dislike him.
12. The name "vitamin" is proposed by Casimir Funk, who suspected that these substances were essential for life.
13. No one knows exactly how many species of animals lives on earth.
14. She said that she had seen the young man and his wife's talking to Jane on he stairs.
15. Robinson Crusoe found his servant on a Friday and so he had called him Friday.
16. The article is said that unless the cattle were in good condition, milk production would never reach such a high level.
17. We have been working and working in the problem for months and i don't think we were likely to solve it.
18. When I came to the place of our meeting there were nobody though I was absolutely sure that we were to meet at the entance of the theatre at a quarter to eight.
19. The hot dog's populatiry begun in St.Louis in 1833 when a sausage peddler named Feuchtwanger slipped one of his franks into a bun.
20. Michigan's rivers and lakes attract tourists that derive pleasure from canoening and water-skiing.
21. I think you should certainly had told us the truth so that we could decide what was the best thing to do.

Answer 1

7: should 
10: has already been noted
11: had disliked
12:was proposed
There is maybe something with the 14th, the end of the sentence I can not understand :(
16: article said that
17: on the problem
18: entrance

These grammar variants are only suggestions (you may have a second look on the original sentences and may find my suggestions correct), the others are correcting mistakes of pronunciation. I hope, I could help a little.