Modal Verbs – What Does ‘You Could Have’ Mean Exactly?

1. Who sent those flowers?- I'm not sure. It could have been your mother.

The above is correct written form. There's a lot of latitude with speaking, of course. Here are some alternatives:

• Who sent these flowers? Couda' been your mother.
• Who sent these flowers? Maybe your mother?
• Who sent these flowers? Could be your mother. <- not likely but understandable. This and other types of "incorrect and slightly odd" spoken responses happens often enough. It can make one think a second longer to fill in the gap or an opportunity to make a joke: "Oh! My mother turned into flowers? So who do you think sent my mother? (haha)"

2) He could have been Prime Minister now if he hadn't decided to leave politics.

The above is correct written form. You could find a place for your "could be" form in spoken language as a parallel, emphatic, emotional voice. The following illustrates this with a similar construction, but with some necessary context:

• Man #1: "It's not so bad...I'm doing well now... I've worked my way up to vice president of the Acme Corp!"
• Man's Tough Friend: "Don't give me that! You could be CEO now if you hadn't messed everything up in the first place."

3) We could have spent today at the beach, but we thought it was going to rain, so we decided not to.

The above third case is not correctly worded as you stated above. It doesn't make sense to say "I could have done X, but since Y I decided not to." That's because the decision was totally volitional. The format is "I would have done X, but since Y I decided not to." So the original sentence above would be more natural as:

• "We [were going to spend | would have spent] [the day | today] at the beach, but we thought it was going to rain, so we decided not to."

To use your original sentence wording ("could have spent"), it would need to be something less within the speaker's control, and perhaps an expression of disappointment like this:

• I could have spent the day at the beach, but Wendy got sick so I had to stay and babysit. Hrmph! (Note that this is also volitional in an absolute sense; yet has a sense of blame.)

To make it your alternative "could be spending" suggests a greater disappointment that is still ongoing and not fully accepted:

• On the phone: "I could be spending the day at the beach, right now, with all my friends, but Wendy just had to get sick and so I'm just stuck here baby sitting. Hrmph! (Note that this is also volitional in an absolute sense; but has a greater sense of disappointment and blame.)

The most mature way to handle the situation would be decisive and volitional:

• "Yes, I know... I was planning on spending the day at the beach with you guys. But Wendy got sick so I'm staying home to take care of her today. I'm certainly not going out if Wendy needs me. Have fun... I gotta go now. Bye."

Compare the following sentences:

Present time

1. It's very dangerous in Metropolis right now. You can be beaten up for just looking at someone the wrong way.
2. It's very dangerous in Metropolis right now. You could be beaten up for just looking at someone the wrong way.

Sentence (1) presents being beaten up as a live possibility, as something that does actually happen to people. Notice that we can analyse for just looking at someone the wrong way, as some kind of condition:

1'. You can be beaten up if you just look at someone the wrong way.

Sentence (2) presents being beaten up as a hypothetical outcome of the wrong look. It is not being presented in the same way as in (1). It doesn't necessarily say that people are being beaten up for looking at people the wrong way - although we might assume that they are.

Could in sentence (2), as already mentioned, represents this outcome as a hypothetical possibility, as opposed to a live one. We could rephrase it as the following hypothetical conditional:

2'. You could be beaten up if you just look(ed) at someone the wrong way.

Remember both of these sentences refer to the present or future time. The way that we interpret you here is quite likely to affect our interpretation of the sentence. It's also quite likely to affect our choice of can or could. If you means a person in general, we are more likely to use can. If you is being used to make the listener imagine themselves in that situation, then we are more likely to use the hypothetical could.

Past time

Let's move forward twenty years. Now if we wish to make the same kind of statement but about Metropolis twenty years ago instead of now, we need to shift the tenses back to indicate past time. Example (1) would now be as in (3):

3. It was very dangerous in Metropolis in those days. You could be beaten up for just looking at someone the wrong way.

Here we see could appearing as the past form of can. This past form of can still implies that people actually were being beaten up for looking at people the wrong way. It is being presented as a live possibility for people at the time. It has exactly the same meaning as can, but refers to a past time. It does not represent hypotheticality.

However, in sentence (2) we already have past tense could, where the past tense indicates hypotheticality. If we wish to keep this hypothetical flavour, but also indicate past time, then we need to shift the tense back further. We need to use a past perfect form as in (4):

4. It was very dangerous in Metropolis in those days. You could have been beaten up for just looking at someone the wrong way.

Sentences (3) and (4) could be construed as the conditionals:

3'. You could be beaten up if you just looked at someone the wrong way.

4'. You could have been beaten up if you ('d) just looked at someone the wrong way.

So if we see a past perfect form could have this most likely represents past time reference plus hypotheticality.

Hope this helps!