1. He uploaded the file on Youtube.

2. He uploaded the file onto Youtube.
3. He uploaded the file to Youtube.

4. He uploaded the file on the website.
5. He uploaded the file onto the website.
6. He uploaded the file to the website.
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Are they all grammatical?

Yes, They are all grammatical.

3 and 6 are the best.

Yes, all of the given sentences are grammatically correct. However, the choice of preposition (on, onto, or to) can subtly change the meaning or emphasize different aspects of the action.

1. "He uploaded the file on Youtube." - This sentence is grammatically correct. The preposition "on" indicates that the file was uploaded to Youtube and is now available for viewing or sharing on the platform.

2. "He uploaded the file onto Youtube." - This sentence is also grammatically correct. The preposition "onto" emphasizes the physical action of moving the file onto the platform of Youtube. It suggests that the file was not originally on Youtube and was physically placed onto it.

3. "He uploaded the file to Youtube." - This sentence is grammatically correct as well. The preposition "to" indicates the direction of the action, suggesting that the file was sent or transferred to Youtube.

4. "He uploaded the file on the website." - This sentence is grammatically correct. The preposition "on" suggests that the file was uploaded to the website and is now available there.

5. "He uploaded the file onto the website." - This sentence is also grammatically correct. Using "onto" emphasizes the physical action of placing the file onto the website.

6. "He uploaded the file to the website." - This sentence is grammatically correct. The preposition "to" indicates the direction of the action, implying that the file was sent or transferred to the website.