1. Now human beings (to kill) our planet. 2. People usually(not to care) about the environment.
3. The builders have (to cut down) a lot of trees in the forest.
4. As a result many animals were to (to die out).
5. When did the destruction of this countryside (to start)?
6. he (to plant) a tree at that time yesterday?
7. he (to plant) down the tree by 6 o’clock yesterday?
8. According to the forecast a disaster (to happen) soon

1. Now human beings are killing our planet.

Explanation: The verb "to kill" should be in the present continuous tense because it is an ongoing action that is happening currently.

2. People usually don't care about the environment.

Explanation: The verb "to care" should be in the present simple tense because it describes a general truth or habit.

3. The builders have cut down a lot of trees in the forest.

Explanation: The verb "to cut down" should be in the present perfect tense because it describes an action that started in the past and has a result in the present.

4. As a result, many animals were dying out.

Explanation: The verb "to die out" should be in the past continuous tense because it describes an ongoing action in the past.

5. When did the destruction of this countryside start?

Explanation: The verb "to start" should be in the past simple tense because it describes a specific action that happened in the past.

6. Did he plant a tree at that time yesterday?

Explanation: The verb "to plant" should be in the past simple tense because it describes a completed action in the past.

7. Did he plant the tree by 6 o'clock yesterday?

Explanation: The verb "to plant" should be in the past simple tense because it describes a completed action in the past. The word "down" is not necessary in this sentence.

8. According to the forecast, a disaster is going to happen soon.

Explanation: The verb "to happen" should be in the future simple tense because it describes a future action or event. The phrase "is going to" is used to express a future action with certainty.