In Youtube

[Xinfu]

Are my sentences correct?

-In Youtube, you can use the direction keys to backward or forward a video or a drama series in the intervals of three seconds, so that if you find it difficult to understand a certain sentence, you can repeat listening to it until you understand what that character is saying. This is a very effective way to learn the language.

[Rusty]

On Youtube, you can use the arrow keys to back up or go/move forward in a video or a drama series in three-second intervals, so that if you find it difficult to understand a certain sentence, you can repeat listening to it until you understand what that character is saying. This is a very effective way to learn the language.'On Youtube' sounds better, referring to the site itself. 'In Youtube' doesn't work unless you add more information, like 'in a YouTube video'.
Backward is not a verb. Forward is a verb, and one of its meanings fits, but I would prefer 'go/move forward in'. It pairs better with 'back up', in my opinion.

Everything else is fine.

[Xinfu]

'On Youtube' sounds better, referring to the site itself. 'In Youtube' doesn't work unless you add more information, like 'in a YouTube video'.
Backward is not a verb. Forward is a verb, and one of its meanings fits, but I would prefer 'go/move forward in'. It pairs better with 'back up', in my opinion.

Everything else is fine.

Thank you. If we can say 'repeat listening', then can we say this?

-John repeats hearing that he is not handsome.

I think the situation here is different. Listening to a song is a conscious action, but hearing other people's comments is just accidental, which might forbid the use of repeat +ing.

[Rusty]

'John keeps hearing' would be the common way to express that.

[Xinfu]

Thank you~