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If I leave, it’s because Bob has arrived.

Does this mean:

  • Bob has not arrived yet. When Bob does arrive (shortly), I may leave.

  • Bob is here now and requesting my attention. Therefore, I may leave shortly.

Or is the sentence inherently ambiguous?

Is there a name for this subjunctive–future-tense ambiguity?

MAJOR EDIT: Apparently, this sentence was even more ambigious than I thought!

The situation: I'm chatting online with someone. I say:

"If I leave, it’s because Bob has arrived."

My meaning: when Bob arrives, I will stop chatting with you, and attend to Bob.

My chat partner's interpretation: Bob has already arrived, and, any moment now, you will attend to Bob and stop talking to me.

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It's not ambiguous. How I understand it, if I see you leave, I'll know Bob has arrived. – Kristina Lopez Jul 18 '13 at 7:11
@KristinaLopez Thank you! That's the meaning I had in mind when I said it... now let's see if others agree. – barrycarter Jul 18 '13 at 7:12
If Bob shows up, I will leave (possibly negative - you do not like Bob); I cannot leave until Bob shows up (neutral) – mplungjan Jul 18 '13 at 7:37
The sentence isn't ambiguous in terms of you leaving when Bob arrives, but the motive is unclear. My initial presumption would be that there's bad blood between you and Bob, and you can't stand to be in the same room as him. That might not be the case. Ambiguity can take on many forms. – J.R. Jul 18 '13 at 10:08
The technically correct (because unambiguous) sentence would be If I leave, it'll be because Bob has arrived. Most native speakers are content to live with the ambiguity. – TimLymington Jul 24 '13 at 23:04

Yes, as it stands the sentence has the ambiguity you describe. There is no subjunctive here in either instance, but your (future) departure is contingent upon an arrival which may or may not have occurred yet.

But in actual use it has no such ambiguity. In real life, as opposed to the pages of a textbook, such sentences are uttered within a context shared by the speaker and hearer. You and your hearer will know whether the Bob you are speaking of is present or yet to come, and in that knowledge the ambiguity is resolved.

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The sentence is not only ambiguous, but also grammatically incorrect.

If I leave, it’s because Bob has arrived.

This sentence has two parts, the first which is conditional in nature. You are stating a condition for an event that will happen in the future: “If I leave” (in the future). You then go on to append this with a statement in the past tense: “Bob has arrived.”

This is where the flaw is: you cannot mix tenses in your sentence. So, you could either say:

I left because Bob had arrived.


I will leave because Bob has arrived.


I will leave because Bob will arrive.

You will notice that in all three sentences the tenses are consistent.

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The sentence is not grammatically incorrect, although as it stands it does refer to a general state of affairs, rather than individual events in the present/future. “If I leave, it’s because Bob has arrived” is perfectly grammatical in the right context. To give an exactly parallel sentence that is much easier to digest in this shape: “If I cry, it’s because you’ve broken my heart” (vel sim). Saying that a current event actually taking place is the result of a completed action is grammatically perfectly plausible. – Janus Bahs Jacquet Jul 21 '13 at 20:07
@Janus: your idea of exactly parallel isn't right, though your last sentence is. 'I do cry, because you broke my heart' is fine; * 'I do leave, because Bob has arrived' is not, because leaving is not a continued action. 'If I leave, it will be because Bob has arrived' would be the purist's phrase. – TimLymington Jul 24 '13 at 23:11
I'm not sure why you are turning the phrases around and adding an unnecessary (and in both phrases very unnatural-sounding) ‘do’ here, nor why you change the tense of some verbs and not others. I don't understand what point you are trying to make at all, I'm afraid. – Janus Bahs Jacquet Jul 25 '13 at 7:29

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