'Simple past tense / present perfect tense' vs. 'would have + past participle'

Question

Although I have asked several questions on "would have + past participle" construction, I still have a few loose ends to tie up.

As Leo adequately pointed out, my doubt is when to use 'simple past tense / present perfect tense' and when to use 'would have + past participle'. (Let's dismiss the irrealis usage of "would have done" in this question)

Consider the example he contributed in his answer to my previous question:

Let's suppose you are a policeman investigating a burglary in the rich neighboorhood and one of the suspects is the neighbour who happens to be rich and you found his fingerprints inside the house where burglary took place.

A jounalist might come up and ask you,"Why would he have stolen the money?", or he could ask,"Why did he steal the money?"

Another example:

A: Why should I believe you? So I can end up in here with you?

B: It's legal. Ask the IRS. They'll say the same thing.

A: I feel stupid telling you this. I'm sure you would have investigated. (Why not say "I'm sure you have investigated"?)

B: I don't need you to tell me where the bear shit in the buckwheat.

A: Of course not. But you do need someone to set it up for you.

-- The Shawshank Redemption (1994)

Answer 1

Not sure if this is exactly what you want to understand. I didn't read through the other posts. Solely going by the examples you gave:

There is still a condition that needs to be met in both instances.

First example, it is uncertain that the guy whose fingerprints they found is the actual burglar. He hasn't been formally convicted of the crime. Therefore, he is still only a suspect until further investigation and prosecution.

"Why did he steal the money?" Means that he did steal it but they want to know the reason.

"Why would he have stolen the money?" Means they want to know the reason this person (provided he is the culprit) wanted to steal it (possibly to use this as evidence in a prosecution.)

Second example, I can't pinpoint because I don't recall this passage but it seems that it's possible someone else investigated and B didn't get the opportunity or something to this same effect. B somehow didn't meet a certain condition in order to actually investigate but you can't make the claim that he actually investigated.

Your sentences are grammatically correct but they don't fit because they conclude that something has happened. The thing is, we don't know for a fact that it did happen.

Hope this helps some.

EDIT: I just read the other posts. Aside from what has already been stated in the other post about 'expression of disbelief'. It is also a matter of intonation, emphasis and context.

Consider this:

/Why would he steal my money?/ - uncertain if he is the culprit

/Why would HE steal MY money?/ - uncertain and stressing ownership

/WHY would HE steal my money?/ - disbelief that 'he' is the culprit

Although all three are written the same way, from a written dialog it would need to be deciphered by context. In a spoken dialog, it would be deciphered by tone and emphasis (and context.)