Here are the completions for each sentence while maintaining their original meanings:

1. If by any chance you come to Hai Phong, drop in and see us.
**If you happen to come to Hai Phong, please drop in and see us.**

2. I regret that I left the job at the bank!
**If only I hadn’t left the job at the bank!**

3. What would you do if you were in my place now?
**If you were in my place now, what would you do?**

4. If you apply by Saturday, you'll get a discount.
**Apply by Saturday, and you'll get a discount.**

5. If you were 5 years younger, what would you do then?
**What would you do then if you were 5 years younger?**

6. Supposing
**the weather had not been so bad, we wouldn't have returned so early.**

7. But for
**his behavior, he will suffer in the end if he keeps behaving that way.**

8. If he will
**continue this way, he will suffer in the end.**

9. If it hadn't
**been for his carelessness, the accident wouldn’t have happened.**

10. If
**you stay up so late, it will absolutely make you tired.**