Here are the correct prepositions filled in the blanks:

1. I was born **in** 1970.
2. Hi, I am an engineer. I go to work **at** 8:00 a.m.
3. I was born **on** August 10th.
4. I go to work **in** a big factory.
5. I start work **in** the morning.
6. I was born **in** 1990.
7. I go to work **at** 5:00 a.m.

(Note: Please note that the instruction mentions filling in "in," "af," or "on," but it seems you meant "at" for the time. "Af" does not appear to be a correct option and is likely a typo for "at.")