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Person A: Where is my car?

Person B: I don't know. Maybe John took it.

  1. Person A: If John took it, he would have let me know.

  2. Person A: If John took it, he will have let me know.

  3. Person A: If John had taken it, he would have let me know.

Q1) In each case person A doesn't know whether John actually took it or not. Which one(s) is (are) correct?

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  • 1
    3 sounds correct to me.
    – user405662
    Commented Mar 7, 2021 at 15:07
  • 2
    (3) is the only idiomatic one. (2) implies that the speaker thinks John may have sent a message which they haven't received yet - an unlikely scenario.
    – Kate Bunting
    Commented Mar 7, 2021 at 15:35
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    "If John took it, he would let me know" is also grammatical
    – Henry
    Commented Mar 8, 2021 at 2:10
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    "If John had taken [the car], he would have told me" is one word shorter and has the same meaning. It sounds more idiomatic to me also, but after living in Italy for almost 40 years my ear is less reliable than it once was.
    – Mari-Lou A
    Commented Mar 8, 2021 at 6:59

4 Answers 4

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Both 1 and 3 are grammatically correct, but I would take them to describe subtly different things.

If John took it, he would have let me know.

This is a statement of expectation. To put it into a wordier form... "If it is true that John has taken it, I expect that he will have left me a message to let me know. I should go and try to find that message". Essentially, this is intended as a conditional statement, asserting that we can determine whether John took it by investigating if John left a message of some sort.

If John had taken it, he would have let me know.

This is a statement of doubt. Wordier: "If it was true that John took it, I would know because he would have let me know. He didn't let me know, so I don't believe John took it". While stated conditionally, it's intended as an argument for why John didn't take it.

If John took it, he will have let me know.

This is, strictly, grammatically incorrect. The grammatically correct form that is the likely intended reading is "If John has taken it, he will have let me know". The issue is that "took" and "will have" do not pair well.

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    I don't think the casual speaker would make that distinction... I can easily imagine someone saying either: "No, if John took it, he would have let me know." OR "Maybe, if John took it, he would have let me know."
    – user3067860
    Commented Mar 8, 2021 at 19:22
  • "took/would have" doesn't sound any better to me than "took/will have". Why do you say the latter is grammatically incorrect but the former is OK?
    – trent
    Commented Mar 8, 2021 at 20:16
  • @trentcl - what sounds best will vary based on regional speech patterns... but the past tense followed by future perfect tense is awkward, at the very least - if the intent is to say that John is going to let me know, it should be "will", not "will have". If the intent is to say that John is likely to have left a message, then it should be "would have".
    – Glen O
    Commented Mar 9, 2021 at 1:17
  • I think both "took" constructions are awkward; that's why I asked -- because I discern no, or very little grammatical difference between "would have" and "will have". If anything, I think "took/would have" is the strictly grammatically incorrect version because "took" doesn't seem to work there either as a simple past or as a subjunctive; but in "took/will have" it scans as a(n awkward) simple past tense similar to "ate" in "If I ate dinner, I don't remember it". I'm just wondering what grammatical rule you think is being violated in "took/will have" but not "took/would have".
    – trent
    Commented Mar 9, 2021 at 2:37
  • @trentcl - the example you gave is comparable to "took/have", not "took/will have". The equivalent to "took/will have" would be "If I ate dinner, I won't have remembered it". Compare with "If I ate dinner, I wouldn't have remembered it", which is equivalent to "took/would have". Where the latter reads as a little clumsy, but valid, the former just doesn't make sense. But let's make it more obvious by using an intransitive verb. "If I ran, I would have won" is a little clumsy, but valid. "If I ran, I will have won" makes no sense.
    – Glen O
    Commented Mar 9, 2021 at 3:07
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In the context of the question posed by A, both 1 and 3 are grammatically correct, but 3 scans better, it has a nice rhythm, whereas A is a little bit abrupt and staccato. If either 1 or 3 are acceptable to you in the story, I would use 3.

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    (2) is possible nowadays with a 'has sent me a text I haven't yet seen' interpretation.
    – Edwin Ashworth
    Commented Mar 7, 2021 at 16:33
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    @EdwinAshworth That's true but I think that it would be more normal to say "If John has taken it he will have let me know". For me 'has taken it' matches the subjunctive better. Consider the similar sentence "If John has set off he will have let me know".
    – BoldBen
    Commented Mar 8, 2021 at 5:31
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Only 3 is good English. 1 is a colloquialism, which is okay in casual conversations but not in print, unless it’s a quote.

"Would have" pairs with "had done"/"been"; "would" pairs with "did"/"were".

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    Do you mean good British English, or is that the same for American English? Where do you come from?
    – Yaroslav Nikitenko
    Commented Mar 8, 2021 at 19:29
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    Conditionals are much more complicated than that in English. Those rules are for cases where John takes it first and lets you know afterwards. The chronology here might be different.
    – Peter Shor
    Commented Mar 8, 2021 at 22:27
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The Zero Conditional (A Truth)

If John takes it, he lets me know.

First Conditional (Real situations that will happen)

If John takes it, he will let me know.

The Second Conditional (An unreal situation: John did not take it.)

If John took it he would let me know.

Third Conditional (Past, did not happen so can't happen.)

If John had taken it, he would have let me know.

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