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I'm wondering what the difference is between these two phrases.

  • What would happen if aliens visited Earth?

  • What would happen if aliens visit Earth?

Am I right in thinking that the first phrase is more hypothetical, and, given that the existence of aliens is indeed hypothetical, it is a more appropriate question?

I can also think of:

What would happen if aliens were to visit earth?

How does this last one compare with the above?

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Both sentences are in the subjunctive voice. With the subjunctive, past and present forms can be used, but neither of them convey a sense of time. They are, in fact, largely interchangeable.

In effect, they are all subjunctive, but the inclusion of words suggesting uncertainty ("what would happen if ...").

What would happen if aliens were to visit earth?

The sentence above is the 'classic' and traditional way of forming a subjunctive sentence that you will find if you Google 'English subjunctive'. But the the subjunctive is characterized most strongly by the uncertainty of the tone of the sentence, not by inflexion of verbs per se. So it can be said in multiple other ways.

What would happen if aliens visited earth?

This is standard way the so-called 'past subjunctive' is formed for all verbs except 'to be'. Although it is called 'past', it does not convey any sense at all that the event has happened before the present. It is largely interchangeable with the present tense here, although some would consider the 'proper' subjunctive form more correct for a sentence beginning with 'what would'.

So, if one really wants to be pedantic (and I don't), it might be better to say:

What will happen if aliens visit Earth?

because that puts 'will' in the future indicative voice, along with present indicative 'visit'. The sentence is no longer subjunctive, but perfectly valid.

All these ways of saying the sentence in effect mean the same thing. It would be wrong to suggest there is any significant difference whatsoever in meaning.

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    Although it is called 'past', it does not convey any sense at all that the event has happened before the present. — would agree there, but I think it does convey that the action happens before the hypothetical situation though. For example, to "What would happen if the stock market were to crash?", you might talk of things in the midst of the event — "chaos in Wall Street", people issuing "sell" orders; if it were "What would happen if the stock market crashed?", you'd focus on things after the event — government stimulus package / redundancies etc. – anotherdave May 25 '20 at 13:18
  • Sorry, but I don't think that's right. As far as I can see, the 'things arising' from the two examples you give would work completely fine with either example. They're both simply asking for hypothetical events following a hypothetical event. I think it's more much likely that as confident English speakers we are able to trick ourselves into seeing distinctions where none actually exist. – fred2 May 25 '20 at 17:19
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What would happen if aliens visited Earth?

So the sentence above could be interpreted as "What would they have done if they were in earth 100 years ago ? How our world would have been impacted nowaday ?"

While this one is different:

What would happen if aliens visit Earth?

Which mean "What will they do if they came in earth nowadays ? For instance will they make peace or declare war?"

Both questions are hypothetical and correct, so none of them is better than the other, it just depend of what you want to ask.

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  • It's the past subjunctive, which does not convey a sense of time at all. – fred2 Mar 8 '19 at 19:11
  • You are right, I should not write "at the present" but "express the present tense" is it better? – Ced Mar 8 '19 at 19:13
  • I'd say it's better to avoid all suggestions that there is a concept of time in subjunctive sentences. They are, in effect, independent of time considerations. – fred2 Mar 8 '19 at 19:18
  • Ok thanks for your feedback I'll remove those suggestions so! – Ced Mar 8 '19 at 19:21

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