1. He is riding a horse.

2. He is riding on a horse.
3. He has to ride a taxi.
4. He has to ride on a taxi.
5. He has to ride in a taxi.
6. He will ride a bus.
7. He will ride on a bus.
8. He will ride in a bus.
9. He will ride the subway.
10. He will ride on the subway.
11. He will ride in the subway.

(Would you check the expressions we cannot use? Thank you.)

All are fine except for #3.

#s 10 and 11 are OK, but #9 is best among these.

Certainly! Here's a breakdown of each expression and whether it is commonly used or not:

1. He is riding a horse. [Correct] - This expression is grammatically correct and commonly used when someone is physically sitting on a horse and controlling its movement.

2. He is riding on a horse. [Incorrect] - This expression is not commonly used. It may sound awkward because it implies that someone is on top of a horse, but not actually controlling it.

3. He has to ride a taxi. [Correct] - This expression is grammatically correct and commonly used when someone needs to take a taxi as a means of transportation.

4. He has to ride on a taxi. [Incorrect] - This expression is not commonly used. It may imply that someone needs to physically be on top of a taxi, which is not the intended meaning.

5. He has to ride in a taxi. [Correct] - This expression is grammatically correct and commonly used when someone needs to travel inside a taxi as a passenger.

6. He will ride a bus. [Correct] - This expression is grammatically correct and commonly used when someone plans to take a bus as their mode of transportation.

7. He will ride on a bus. [Correct] - This expression is grammatically correct and commonly used when someone plans to board a bus and be a passenger.

8. He will ride in a bus. [Correct] - This expression is grammatically correct and commonly used when someone plans to travel inside a bus as a passenger.

9. He will ride the subway. [Correct] - This expression is grammatically correct and commonly used when someone plans to use the subway as their mode of transportation.

10. He will ride on the subway. [Correct] - This expression is grammatically correct and commonly used when someone plans to board the subway and be a passenger.

11. He will ride in the subway. [Incorrect] - This expression is not commonly used. It may suggest that someone will be physically inside the subway system itself rather than being a passenger on a subway train.

In summary, expressions 1, 3, 5, 6, 7, 8, 9, and 10 are commonly used and grammatically correct. Expressions 2, 4, and 11 are not commonly used or may convey a different meaning.