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Group of sentences:

1) Where are you off? Don’t you know there’s an English movie this afternoon?

2) Where are you off to? Don't you know there’s going to be an English movie on this afternoon?

3) Where are you off to? Don't’ you know there will be an English movie on this afternoon?

Questions:

1、 Since where can function as an adv, is "to" necessary there?

2、Which tense do you like best? (My opinion is 1 is best)

Thanks in advance!
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When you mean "leaving" then "off to" stays together. (It's not the same as "Where did you get off the bus?" where "off of" would be nonstandard.) So you need #2 for that part.

As for tenses, either 1 or 2 make sense to me. #3 doesn't strike me as incorrect, but it doesn't sound natural. With #1, it sounds like it's already early afternoon, or close to it. #2 you could say first thing in the morning.

I think you need something like "there's an English movie showing this afternoon" or "there's an English movie on this afternoon." You have that in #2 and #3, but not in #1.
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> Where are you off to?

You're adding the to in order to ask for his/her destination.
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Comments  
So on here is prescribing the stat of the movie rather than a prepesition collocating with this afternoon (on this afternoon).

to and on are necessary there.

It's very helpfulof your comments! Than you!!Emotion: smile

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And how about this sentence:

Where are you off to ? Don’t you know there’s an English movie on in 2 hours?

Is the tense proper?
And how about this sentence:

Where are you off to? Don’t you know there’s an English movie on in 2 hours?

Is the tense proper?

Sounds completely natural! Good job. Emotion: smile (One minor comment - no space before the ?)
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Grammar Geek
(One minor comment - no space before the ?)

Sure, no space!Emotion: embarrassed Thank you!Emotion: smile
Marius Hancu> Where are you off to?

You're adding the to in order to ask for his/her destination.
I've got it. Thank you![C]