Here are the correct tenses for each sentence:

1. If it costs too much, I **will buy** a small one.
2. If you **drove** more carefully, you wouldn’t have so many accidents.
3. What **would happen** if there **were** gravity?
4. If I **make** that mistake again, my teacher **will get** angry with me.
5. If I spoke English, my job **would be** a lot easier.
6. If he **went** to London yesterday, he **met** his old friend.
7. Would you mind if you suddenly **won** half a million pounds?
8. I **will lend** them some money if they **ask** me.
9. If we **knew** who he was, we **would invite** him to speak at our meeting.
10. If they had invited us, naturally we **would go** to the party.

Let me know if you need any further assistance!