Unless the plane encounters unexpected turbulence, it ------- in Iceland shortly before 5 P.M.

In this conditional sentence using "unless", should the blank be filled with:

1) is arriving, 2) will arrive


Are both 1 and 2 acceptable?

If one choice is more correct or more favorable than the other, can someone please explain why they think so?

Thanks. :-)

  • 1
    Also consider the simple present: it arrives. – tchrist Dec 3 '14 at 3:32

Compare the following:

  1. Unless you apologize, I'm going to punch you in the nose.
  2. Unless you apologize, I will punch you in the nose.
  3. Unless you apologize, I'm punching you in the nose.

All of the above are acceptable English and express the same condition and meaning with a difference only in register (formality).

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Both are correct, but they don't mean quite the same thing.

"Is arriving" (the traditional "present progressive") need not only reference the present time. It can also reference an action arranged for the future.

"Will arrive" (the traditional "future tense") references not just an action in the future but also an assumption with regard to the future.

So the difference is between whether you are thinking about the arrival as an "arrangement" or an "assumption". The conditionality of the "unless" clause does not affect this.

You could also use "should arrive" (emphasising the obligation of the airline), or plain "arrives" (emphasising action set by a timetable or schedule).

This is a good list of uses for the English tenses which helped me:


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  • Great. Thank you! That website is an excellent resource. :-) – timjames Dec 4 '14 at 5:20

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