Sure, here are the completed sentences:

1. Everyone ran right after the tsunami siren went off.
→ As soon as the tsunami siren went off, everyone ran.

2. They predict the hurricane won't hit the city, but the residents still get prepared for the worst.
→ Although they predict the hurricane won't hit the city, the residents still get prepared for the worst.

3. The guidelines for evacuation will be useless if people don't read and follow them.
→ Unless people read and follow the guidelines for evacuation, they will be useless.

4. Angelina is too young to go to a party for adults.
→ Angelina isn't old enough to go to a party for adults.

5. It was such a strong tsunami that thousands of people were killed.
→ The tsunami was so strong that thousands of people were killed.

6. The firemen spent three days putting out the forest fire.
→ It took the firemen three days to put out the forest fire.

7. Let's go to a shopping mall instead of surfing the online stores.
→ Why don't we go to a shopping mall instead of surfing the online stores?

8. I have to prepare meals on my own, so I often buy groceries.
→ Since I have to prepare meals on my own, I often buy groceries.