Question

1. I don't know who to say.
2. I don't know who to say to.
3. I don't know whom to say.
4. I don't know whom to say to.
5. I don't know who to say about.
6. I don't know whm to say about.
7. I don't know to whom to say.

(Which ones are grammatical?)

8. He decided to go abroad.
9. He determined to go abroad.
10. He made a decision to go abroad.
11. He made a determination to go abroad.

(Are the four sentences all correct and the same?)

12. Have you ever ridden a bike?
13. He rode a bike in the park yesterday.

(Are both grammatical?)

Answer 1

John, in #'s 1-7.... I don't think you want to use "say"... I am not sure what you are trying to say there.

We would say , "I don't know whom to tell." or "I don't know what to say."

The rest of your sentences are correct.