The corrected sentences are:

1. Despite being late for the discount products there, he was in a hurry, but he stopped to help the old man who fell down at the market.

2. While he was in a hurry, he stopped to help the old man who fell down at the market.

3. Nam is a student. He doesn't usually go to school late.

4. I saw Mark get off the bus and then go straight to the supermarket.

5. You should reduce the amount of sugar you consume.