Давайте разберем формы спряжения глаголов "to be" и "to have". Эти глаголы являются неправильными и имеют свои уникальные формы в зависимости от времени и лица.

Глагол "to be":

• Настоящее время: am, is, are
• Прошедшее время: was, were
• Будущее время: will be

Глагол "to have":

• Настоящее время: have, has
• Прошедшее время: had
• Будущее время: will have

Теперь давайте вставим соответствующие формы этих глаголов в предложенные вами предложения.

1. The letter is in his hands.
2. He has a letter in his hand.
3. We have Rousseau's book Letters from the Hills.
4. Our city is very big.
5. We have a small house.
6. Our works have a new conveyer.
7. The new carriages are blue and yellow.
8. The locomotive is black.
9. This building is in that part of the city.
10. This place is very famous.
11. The foreign workers are in that carriage.
12. The tower which we see there has a clock as you see, the clock is very big.
13. These men are our new workers.
14. Over a hundred people are now in this building.
15. These sheets are white.
16. We often have foreign visitors here.
17. Our city has twenty libraries.
18. The gate is open.
19. The works are near the city.

Таким образом, мы видим, как правильно использовать глаголы "to be" и "to have" в разных предложениях, следуя правилам их спряжения.