Difference between can't and couldn't

Question

I am doing exercises for English and I am asked to match 1 and 2 with A and B, but the sentences look very much alike to me.

1: "I don't believe the plan succeeded"

2: "It wasn't really possible for the plan to succeed"

A: "The plan couldn't have succeeded"

B: "The plan can't have succeeded!"

Can anybody distinguish the meanings of these sentences?

Answer 1

In these cases, "can't have" is being used to denote disbelief or incredulity, while "couldn't have" is being used to denote an impossibility.

Can and could are modal verbs, and represent present and preterite forms respectively. The preterite is not necessarily used to refer to past time, such as in this case, where it's being used in the subjunctive mood, expressing the possibility of something occurring.

Answer 2

!reminder: there are no hard grammatical rules in English. The info herein references commonly-known phenomena that are often taught to children learning General English in an effort to bring them closer to language mastery.

Hypothetical statements are broken into two parts: (1) condition and (2) result. Ordinarily, in English it is said that we move back a tense during a hypothetical condition (will be allowed/able to -> can; can -> could; could -> could have -> could have -> could have had). If that's not possible, you might be able to figure out the tense based on the result clause (e.g., "If I could, I would." past/present result; "If I can, I will" future result).

Ability/Permission:

can is used for ability--having the skill or capacity to succeed--and/or permission--being allowed (as if by your parents). The listener must decipher the sense: in almost all ethical situations, word sense is defined and re-defined by the speaker.

can [not] versus could [not]

• outside of the future hypothetical condition,can't means 'am unable/disallowed to'.
• outside of the hypothetical tense couldn't means 'was unable/disallowed to'.

can/could in the hypothetical condition

• in future-tense hypothetical condition, usually can is used because it is grammatical; however, people sometimes use others tenses to increase clarity or because they are less-literate or more colloquial.
• in present-tense hypothetical condition, usually could is used because it is grammatical; however, people sometimes use others tenses to increase clarity or because they are less-literate or more colloquial.

In formal grammar, in the present-hypothetical condition, only the past-tense is used.

Let's look at some examples to clarify.

Examples:¹*

1. He could go to the concert if you'd given him your ticket.
2. He could go to the concert if you gave him your ticket.
3. If he called me, I couldn't hear.

(1) Is probably most grammatical. It can carry the same meaning as "He could go to the concert if you'll give him your ticket," which is also completely grammatical.

(2) Also likely but more colloquial: gave should mean "had given" and could mean "will give". gave isn't formally grammatical--it doesn't make any logical sense--, but in spoken English it would be used by most speakers because it's less difficult to say than "had've" or "have'd".

(3) Similarly, 'has' was omitted in the usual fashion. Carrying the obvious, intended meaning: "If he has called me, I was unable to hear," in the actual sense. And if the speaker believes there was no call: "If he had called me, I was unable to hear." I think this is important because you can see how words effect empathy. We can't understand whether or not the speaker believes a lie was committed due to the colloquial omission of proper, common tense.

Ambiguity through colloquialism

If we get into spoken English and investigate hypothetical conditions, could and can are often treated as completely ambiguous and interchangeable so be careful. That is to say, they could mean:

• 'am unable to...'
• 'wouldn't be able to...'
• 'won't be able to...'
• 'wouldn't have been able to...'

We can also use continuous tenses in the presumptive mood, which walks the line between being hypothetical and real. Only loosely connected and hardly worth mentioning (only worth mentioning here to get a more complete list), examples follow:

1. If my duck hadn't been strutting, it would no have been eaten.
2. If your daughter has been calling my son, we need to change our number.
3. When my wife is working, I might eat poorly.
4. If Drake will be racing his hotrod, we'll all be in attendance.

It depends on the speaker's speaking style, and is usually deciphered by what follows the condition. Usually, this does not create ambiguity because hypothetical statements appear in the context of their analogous actual circumstances. It is the listener's responsibility to decipher the tense, and it is the speaker's responsibility to explain the tense, if necessary; furthermore, in the case of skilled writing, the author should decide whether or not explanation should be required.

Etymology?

I can't tell you why people sometimes jump back a tense when making unreal statements. It's a useful training tool, but it falls apart very quickly when applied.

I have noticed that unreal clauses are sometimes completely interchangeable with real clauses, though. This is my best guess as to why this transformation drill is so useful for adapting unreal language:

With extremely rare exception, modals don't change form when put into the future tense: we can say there are no "future tense" modals (or there are no present-tense modals if you find that more convenient). Anyway, that's probably why people jump back a tense when using unreal language (because unreal language usually prepends modals).

Because of the ambiguity, you should be ready to ask people if they're being hypothetical. During important conversations, this question should appear very often.

Implications

I wanted to add this important section.

Colloquial language is very valuable for teaching, and it has helped the US, specifically, to decrease levels of illiteracy. I have seen convincing evidence that people who use more colloquial language have broader vocabularies and a stronger propensity for poetry.

On the other hand, it substantially damages business and political communications, adding a profound ambiguity to communications, to be overly colloquial. I often find that adult Native English speakers do not have a solid understanding of the perfect tense. Being a University English teacher, I rarely encounter a high school graduate that can correctly describe the perfect past tense, for example.

You can probably guess why it worries me when someone is unable to effectively disambiguate tenses: it poses a real danger in business and in familial society. It is my position that people who always use colloquial language eventually lose their mastery of entire tenses; therefore, tenses should be emphasized more in late, compulsory education, in Native-English-Speaking countries.