Here are the corrected tenses for the provided sentences:

**II/ Complete the sentences:**

1. From 1665-1667 Isaac Newton **laid** the foundations for his famous **work** in Mathematics and Physics.
2. If there weren’t **gravity**, no one would weigh anything at all.
3. Both the United States and Russia **chose** their **astronauts** carefully.
4. Astronauts **discover** the earth in space capsules are weightless.
5. There are many planets in the **universe**. We cannot count them.
6. The pull of the earth is **gravity**.
7. Have you ever dreamed of **flying** into space miles and miles above the earth?
8. All of them were happy about their **flight**, but glad to return to the earth.
9. Gravity makes balls **thrown** into the air fall back down.
10. All the **astronauts** who are orbiting the earth are weightless.

**III/ Correct tenses:**

1. If it costs too much, I **will buy** a small one.
2. If you **drive** more carefully, you wouldn’t have so many accidents.
3. What **would happen** if there **isn't** (or **wasn't**) gravity?
4. If I **make** that mistake again, my teacher **will get** angry with me.