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I'm going to show you three examples.

1. A - Have you got any change?
    B - Do I have change? What for?
    A - For the vending machine!
           I would have thought that was pretty obvious.
: I think it's sufficient to just say,
  "I thought that was pretty obvious."
  because literally 'I' thought so in the past, which is a simple fact, but now I don't.
  And "I would think that as pretty obvious." is possible in the context?
  If so, what are the differences among them?

2. Let's say there are two kids playing in the garden
     and their mum comes out to tell them in for dinner.
     But the kids have gone somewhere, so she starts going around for them.
     In some minutes, she finds the two and says.
     "I would have known."
     Why does she talk like that? What's the nuance?
      I think it will do to say,
    "I knew you would be here."
      I wonder if "I would know" is a possible subsitute for the highlighted sentence.
      If it could be, what are the differences among the three?

3. A - Was the party great?
     B - I don't / wouldn't know. I wasn't there.
: I think if B goes for "I don't know",
   it will mean he's just saying the plain fact that he wasn't there.
  What if B replies, "I wouldn't know"?
  What's the difference?
   And is "I wouldn't have known" possible here?
   If so, what are the differences?

I hear 'would do / would have done' structure is closely related to a hypothetical if,
but I can't seem to figure it out.
I was guessing 'would have done' might mean speculating about the past,
I've come to think it can't explain 'would do' option.

Help me out with this.
Thank you all.

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4  
The second example doesn't make sense. Seems like "I should have known" would make more sense in that case. –  fuandon Jul 30 at 20:26
2  
As @fuandon says, the second example is just plain wrong. In the other two, using the past "conditional" form slightly "distances" the speaker from the the context, which makes it more formal. In principle it also makes the utterance less "blunt" to the person addressed, but in fact in example #1 the net effect is to "elevate" the speaker, making him seem even more condescending to the addressee. Effectively it's "I haven't actually thought about this because it's too trivial for me to bother with - but if I did, this is what I would think". –  FumbleFingers Jul 30 at 20:35
    
@FumbleFingers Thank you! Is your explanation true for 'would have done' structure? I wonder if 'would have done' is more 'polite' or 'distant' than 'would do'. If it is, 'would have done' has nothing to do with the past? –  QNC Jul 30 at 20:49
    
The second example is actually as fine as the others. Here, would expresses that the kids notoriously disappear to that place just before dinner time: "I would have known that you are here, because you will always come here just when I'm about to call you." –  Hans Adler Jul 30 at 21:04
2  
@Hans Adler: I think you are very mistaken. Sure, Anglophones are pretty sloppy about tenses, but I can't imagine even some oddball dialectal speaker saying "I should have known that you are here" - it would always be wrong (s/b "...that you were here"). OP's example #2 seems vanishingly unlikely to me. –  FumbleFingers Jul 31 at 2:30

3 Answers 3

  1. I’m sure all of these variations are commonly used and understood to mean roughly the same thing. “I would have thought…” essentially means “I believe you should have understood what I meant,” or “that should be obvious.” “I thought that was obvious” is a simple declaration that sounds more like the speaker actually gave the matter some thought.

  2. “I should [not “would”] have known,” means that she realizes, after finding the boys, that she should have expected to find them where she did. “I knew you would be here” is similar, but means that she went straight to wherever she found the boys based on her knowledge of their habits. Both phrases might imply some disapproval of their being there. “I would know” (or “I should know”) is not a possible substitute for your sentence. As a complete clause, it’s more like “you can take my word for it,” or “take it from me,” implying some experience of a situation, often said ruefully or with a little chagrin.

  3. “I don’t/wouldn’t know”: There wouldn’t be much reason to answer “I don’t know”; it’s more direct to just say that you weren’t there. “I wouldn’t know” might be employed where there’s some displeasure, like if B thought he should have been invited to the party but wasn’t. The “would” in “I wouldn’t know” is present tense; “I wouldn’t have known” is not possible here because it puts it in the past tense.

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"I would think" and "I would have thought" are instances of the common use of the conditional to soften assertions. This can be useful in establishing plausible deniability or saving face in case the assertion proves to be incorrect. It can also help reduce the harshness of a correction (that is, saying "no, three squared is nine" is blunt and might offend, but "I would have thought that three squared is nine." is less blunt and more likely to be accepted.)

Stephen Pinker touches on this some of his writing, and in this talk he discusses how it is related to people's managing their relationships with one another: http://www.ted.com/talks/steven_pinker_on_language_and_thought

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  1. I am not completely sure myself, but I think this is probably I thought with an added tentative aspect: I would/should have thought. In this situation it's almost equivalent to say I think. If you add the tentative aspect to that you do in fact get I would/should think. (The reason I say I am not completely sure is that there might be another equally valid or better explanation. There are so many use cases for English modals that they often overlap.)

  2. I would have known, with an emphasis on would is similar to children will play or things will break, but backshifted to the past. This use of will (or in the past: would) expresses a habit or feature, often with a connotation of disapproval. Obviously I would have known isn't precisely the same use, but it comes close. I knew you would be here doesn't carry the connotation that this knowledge is based on the children's habits, and the (possibly just playful) disapproval of those habits. I would know doesnt work. It's derived in the same way from I know, which doesn't make sense. Of course she knows now where they are, as she is now in the same place. This has nothing to do with their habits. No prediction is involved.

  3. I wouldn't know means something like this: I don't know, for a specific reason that I want to stress without necessarily saying yet what it is. Some other sentences that could replace I wasn't there: As it was the first time someone invited me, I wouldn't know what to compare it to. - As usual, they threw me out before anything substantial happened. - The only thing I noticed was that Penny was there. Again, this is similar to kids will play, only backshifted in time and this time it's not about a habit but more generally about something notorious. I wouldn't have known would be the result of doing the same operation not on I don't know but on I didn't know. I didn't know makes no sense in this context, and neither can I see another reason why I wouldn't have known should make sense.

PS: As there seems to be some confusion about 2: For present tense this use of will is explained on p. 194 of The Cambridge Grammar of the English Language:

Propensity

[40] i He will lie in bed all day, reading trashy novels.

ii Oil will float on water.

Here we are concerned with characteristic or habitual behaviour of animates or general properties of inanimates. A simple present could be substituted with little effect: this use is therefore fairly sharply distinct from futurity, though in many cases there is a connection through conditional consequence - compare [ii] with If you pour oil on water it will float. Strong stress on the auxiliary conveys the speaker's emotive response to the situation - usually exasperation, disapproval, resignation, or the like: He WILL pour the tea-leaves down the sink.

Now obviously in a past tense context in a novel we would expect to read: He WOULD pour the tea-leaves down the sink.

Without any special exasperation marking the mother might say: "I knew you would be here because you always run away and come here before dinner." With exasperation marking as described above, this becomes: "I knew you would be here because you [always] WILL run away and come here before dinner." It is then natural to apply the same transformation also to the first half of the sentence to make the sense of exasperation even more obvious - even though in this case it's a bit more general because the exasperation doesn't refer to her knowledge but to what there is to know: "I WOULD have known you would be here because you WILL run away and come here before dinner."

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Thank you for the detailed answers. About 1, now I'm thinking that 'would think' is more distant or polite than 'I think', and 'would have thought' is more than 'would think'. Am I following you well? And unfortunately, I couldn't understand the answers to 2, 3, because I've not figured out a connection between the negative feeling of 'will' and 'would have done' structure. –  QNC Jul 30 at 22:46
    
I would think is more distant than I think. I would have thought is in exactly the same relation to both I thought and (theoretically, but with this verb it doesn't make sense) I have thought. The difference between I would have thought and I would think is a priori the same as between I thought and I think: past and present. –  Hans Adler Jul 30 at 23:05
    
I thought carries the connotation that I probably no longer think so, hence it's probably not true. The same obviously applies to I would have thought. –  Hans Adler Jul 30 at 23:06
    
As to 2 and 3, I don't quite understand what you are asking. Do you understand sentences such as "He will leave without paying"? This is not about the future (though it could be, especially with a different emphasis), it's about habitual behaviour. The next step is to shift this to past tense: "He would leave without paying." That's about a habitual behaviour in the past. So much for the easy part. –  Hans Adler Jul 30 at 23:14
    
Now to the hard part. What the mother says isn't an exact case of this phenomenon. She generalises it to a similar case. This is so sloppy that with I will know it would be unacceptable because then the grammar would be too transparent. But with I would have known it's sufficiently complicated that we lose track of the grammar sufficiently to make this acceptable. I really should have made this clearer, but I forgot the technical term for this phenomenon and can't find a reference right now. Oh, and the similarity to "I should have known" probably helps as well. –  Hans Adler Jul 30 at 23:17

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