[Grammar] I knocked (on) the door for three minutes before (until) someone answered it.

[LiuJing]

I knocked on the door for five minutes until someone answered it.

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Two questions if I am allowed to ask.

1---- Is the 'on' optional?
2---- Can the 'until' be replaced with 'before'?

Thank you.

 

[emsr2d2]

I knocked on the door for five minutes until someone answered it.

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Two questions if I am allowed to ask.

1---- Is the 'on' optional? No.
2---- Can the 'until' be replaced with 'before'? Yes, and I actually prefer "before" when used with the time frame. See more info below.

Thank you.

My basic responses are above in red. With #2, as I said, I prefer "before" when used with "for five minutes".

I knocked on the door for five minutes before someone answered.
I knocked on the door until someone answered.

 

[LiuJing]

My basic responses are above in red. With #2, as I said, I prefer "before" when used with "for five minutes".

I knocked on the door for five minutes before someone answered.
I knocked on the door until someone answered.

Thank you for the distinctive explanation, and I've noticed you used 'answer' rather than 'answer it'. Does it mean my version 'answer it' sounds a bit strange and unnatural?

 

[emsr2d2]

Thank you for the distinctive explanation, and I've noticed you used 'answer' rather than 'answer it'. Does it mean my version 'answer it' sounds a bit strange and unnatural?

It's not strange or unnatural, it's just unnecessary. You've already said that you were knocking on the door. It's clear that the door is what someone answered.