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?)
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.
1. I don't know who to say. - This sentence is grammatically incorrect. The correct form is "I don't know whom to say."
2. I don't know who to say to. - This sentence is grammatically correct.
3. I don't know whom to say. - This sentence is grammatically incorrect. The correct form is "I don't know whom to say to."
4. I don't know whom to say to. - This sentence is grammatically correct.
5. I don't know who to say about. - This sentence is grammatically correct.
6. I don't know whom to say about. - This sentence is grammatically incorrect. The correct form is "I don't know who to say about."
7. I don't know to whom to say. - This sentence is grammatically correct.
Regarding the second set of sentences:
8. He decided to go abroad. - This sentence is grammatically correct.
9. He determined to go abroad. - This sentence is grammatically correct.
10. He made a decision to go abroad. - This sentence is grammatically correct.
11. He made a determination to go abroad. - This sentence is grammatically correct.
All four sentences have the same meaning, expressing the idea that he chose or resolved to go abroad.
Regarding the third set of sentences:
12. Have you ever ridden a bike? - This sentence is grammatically correct.
13. He rode a bike in the park yesterday. - This sentence is grammatically correct.
Both sentences are grammatically correct and convey the idea of someone riding a bike, with Sentence 12 inquiring about someone's past experiences with riding a bike, and Sentence 13 stating a specific instance of riding a bike that occurred yesterday.