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A.The interviewer: You just got out of the Navy. How was
that experience?
The interviewee: I was in for nine years, so it
couldn't have been all that bad.

I was taught that could is the past of can, so I am wondering why the interviewee didn't use couldn't instead of couldn't have, since he's talking about something that happened in the past. Can I replace couldn't have with couldn't? What's the difference between them?

B. She told the man who stood her up to get lost, telling him that he only gets one shot with her, and he blew it!

Imagine what this woman would have done with a boyfriend who ____(cheats on/ cheated on) her.

I have three questions about the passage above

1. Should I fill in the blank with cheats on or cheated on, or either? Why?

2. I was taught that would is the past of will, so why doesn't the writer use would instead of would have? Can I replace would have with would? since the scenerio is set in the past. What's the difference between them?

3. The passeage is written in the past. Why does the writer insert a phrase in the present telling him....he only gets.... between the sentences?

Thank you for answering my questions!!
Comments  
have been
is normally used in
unreal or imaginary situations in the past

The tense isn't that important with the modal verbs, the degree of doubt is more important in their choice. Thus I don't think you can replace the have been versions here without removing a large part of the impossibility/improbability/imaginary of the situation.

But check this other thread here:
http://www.englishforums.com/English/CouldDoHaveDone/cmmdn/Post.htm

Also:
http://grammar.ccc.commnet.edu/grammar/conditional.htm

Also check the discussion here:
"Would be" or "would have been" in past?
http://tinyurl.com/s3fau
BTW, JerrySin that discussion is meEmotion: smile
Example A.

First of all, a modal is followed by a bare infinitive. Therefore, could is followed by have or be or any other bare infinitive. have is followed by a past participle, for example, been. So if you literally substitute couldn't for couldn't have, you will get the ungrammatical combination couldn't been.

Let's suppose, however, that you adjust verb forms and substitute couldn't be for couldn't have been.

couldn't can mean several things. couldn't can mean it was not possible, but it can also mean it would not be possible. In the first case it has the sense of the past; in the second case it has the sense of a hypothetical present tense, that is, it places the whole idea into the world of conjecture, saying, in effect, it is not possible in any possible world, under any possible circumstances.

I tried to reach the top shelf, but I couldn't. (past sense: I was not able / It was not possible)
I'm sorry, but I couldn't do that for you; it's dishonest. (hypothetical: I would not be able / It would not be possible / It is not possible no matter what you say or do)

The combination could be / couldn't be takes the hypothetical meaning when be is used in its stative sense.

Things could be better. (It would be possible for things to be better. In some alternate possible world I imagine things, and things are better.)
Things couldn't be better. (It would be impossible for things to be better. In no alternate, hypothetical, imagined world are things better.)
I couldn't be more pleased. (It would be impossible for me to be more pleased. There is no set of circumstances, no imagined, hypothetical situation, in which I see myself more pleased than I am now.)

Note that these statements are not in the past tense. They do not say:
It was (then) possible / impossible for things to be better.
It was (then) impossible for me to be more pleased.


But rather:
It would now be possible / impossible for things to be better.
It would now be impossible for me to be more pleased.


>>> In order to make the idea be relate to the past (be better, be pleased, be bad), have been must be used after could(n't).

Things could have been better. (This would be possible: Things were better. / A set of circumstances can be imagined such that things were (then) better.)
Things couldn't have been better. (This would not be possible: Things were better. / A set of circumstances cannot be imagined such that things were (then) better.)
I couldn't have been more pleased. (This would not be possible: I was more pleased. / A set of circumstances cannot be imagined such that I was (then) more pleased.)

Apply this to Example A.

It couldn't be all that bad. (This would not be possible, in any hypothetical world: It is so bad as that. / A set of circumstances cannot be imagined such that it is so bad.)
It couldn't have been all that bad. (This would not be possible, in any hypothetical world: It was so bad as that. / A set of circumstances cannot be imagined such that it was so bad.)

Note the usage in context:

At work:
-- I have to attend a funeral for my boss's pet pig.
-- It couldn't be that bad. You'll manage, I'm sure.

-- I had to attend a funeral for my boss's pet pig.
-- It couldn't have been that bad. I'm sure you managed.

While driving:
-- Look! There's Mount Shasta in the distance.
-- It couldn't be Mount Shasta. We're in Los Angeles!

-- Did you notice that we passed Mount Shasta an hour ago?
-- It couldn't have been Mount Shasta. We've been in Los Angeles all the while!


___
-- How does Mary like the book you gave her?
-- She couldn't be more interested. She's reading it now.

-- How did Mary like the book you gave her?
-- She couldn't have been more interested. She has already finished reading it.
____

-- What's that strange shape in the sky?
-- It could be a flying saucer!.
-- No. It's more likely an airplane.

-- Did you see that strange shape in the sky last night?
-- Yes. It could have been a flying saucer!
-- I don't think so. It was probably just an airplane.


CJ
Teachers: We supply a list of EFL job vacancies
To Cali:

I really appreciate you for giving me such a detailed explanation!!

I couldn't be more grateful!!!

Acoording to your explanation, for B I should use cheated on instead cheats on, right?

Could you explain to me why the author used gets instead of got in the past scenerio?

Another doubt just set in as I finished reading your response

Since would/would have, could/could have , might/ might have are all the past forms of will, can and may, how do you know when to use would, could , might and would have, could have, might have in the past?

Thanks for answering !!!!!
Example B.

The author inserts the present tense (he only gets one shot with her) into a past tense sentence as a stylistic device to indicate that her rule is 'an eternal truth'. This is her way of judging a boyfriend for all time, not just in the past. The past could have been used, but it would not have had this jarring effect of emphasizing how important this rule is to her.

To continue in the same style set up in the beginning of the passage, you would have to use the present again (cheats on). (But it would be completely grammatical and more usual to use the past.) If I said it myself, I would use cheated on, but that would break the parallelism in the passage as a whole. There is no one correct answer here, in my opinion.

CJ
how do you know when to use would, could , might and would have, could have, might have in the past?
This is a very, very big question. In short, you need to continue your studies of English, and all of these issues will be cleared up for you one day! There are multiple uses for each of these combinations, and it is not really possible to answer in a single post. Entire books have been written on the subject!

Do keep this in mind, however: would, could, should, and might are past tense forms, but they are very frequently borrowed into present time. The historical origin of these words centuries ago is from the past tense, but in modern English the original past tense meaning is not always relevant. For some situations, would is like the past of will, could is the past of can, and so on, but for other situations (perhaps even the majority of situations), these words are treated almost as if they were completely separate and unrelated words with separate and unrelated meanings. should is the most obvious case. In modern English it is hardly ever used as the past of shall, which is its historic origin. It is treated as a separate word with a separate meaning from shall.

CJ

Although it is not directly related to the use of have after a modal verb, you may find this post instructive:
Modals
Students: Are you brave enough to let our tutors analyse your pronunciation?
CalifJim has posted a lot of right stuff here.
I think this should be in the sticky threads at the top.
I think I know how to use them in various situations. One of my students inquired about such a question and I didn't know how to explain to him , in that I often hang out with my friends from the States and Canada, I learned English spontaneously from the way they speak, without grammar. If a non-native speaker asks you such a question, how would explain the grammar to him?