Learn English – The usage of “If I was going to eat” and “If I am going to eat”

I have answered a similar question of yours here: He mustn't/couldn't have been

To answer succinctly your question, you can use can't have and couldn't have indifferently.

I know that online there are websites that infer can't have = 99% certainty, whereas mustn't have (which is non-standard English)= 95% can't have vs.couldn't have But how do you measure 99%? And why is mustn't fixed at 95%?

b) If I use couldn't instead of can't here, will it decrease the probability of my supposition?

When speaking, listeners will understand how certain you are by your facial expressions and voice intonation. Some will argue that using couldn't tentatively increases uncertainty. I think this is subjective, there is no fixed rule. Use whichever you're most comfortable with, and that includes mustn't have

If however, you are planning to sit an exam in the future, choose between can't have or couldn't have in your writing. Every time.

These are all meaningful, but the meaning is changed and the context in which they could be used would be changed. "Would" is more often used to discuss expectations (that one feels certain about) in projected courses of action, rather than hopes or expectations in the current situation. You wouldn't use "would" there except in limited circumstances, when you are very confident in your predictions. I think a lot of this falls out of the use of "should" to describe model behavior or what is 'right' (like def. 3 here). Incidentally, I think (without proof) that that is why we don't use "should" to describe expectations that go awry, like the train being late: we certainly don't think that our hopes or expectations ought to be denied!

1. with "should" is a prediction; with "would" the speaker is certain enough to, for instance, discount the possibility of it not having been received. ("They would have it by now, so that can't be the reason they haven't responded...")

2. "should" states a probable expectation, while "would" is more like a certainty in a hypothetical plan ("We would get there by six, which would give us time to change before dinner...")

3. is tough. "We wouldn't have to wait much longer" is an acceptable utterance with an appropriate context: if I'm making a case for my plan (staying in line) over your plan (leaving the line). The implied full thought is "we wouldn't have to wait much longer [if we agreed to stay]." Here we are discussing the expected-to-be-certain details of possible plans, rather than stating our hopes about the future.

4. would work if you'd said "there were no reports of delays, the train would be on time [and that is why I am worried that your sister hasn't arrived yet]". This is related to sentence 1; I am stating a prediction about which I feel certain enough that I discount the train being late as a possible reason that your sister is delayed, and start worrying that something else could have happened to her.

5. Meaning is completely changed--we don't use "should" to emphasize things that happen contrary to expectation. Sometimes "The train would be late!" (with emphasis) is used expressing frustration that the train is late. The resulting statement is whining. A more full example: "The train would be late on the day I have an interview! This always happens to me!"

So: the distinction as I see it is pretty much what your edited addition says. "Should" suggests a more tentative attitude than "would," which may be from genuine uncertainty, or from an attempt to be more polite or more emotionally removed.

Both "should" and "would" are used to discuss an event about which the speaker is not completely certain or confident. But "would" seems to have the implication of talking about the details of a hypothetical world, future course of action, proposed plan, etc. "Should" is used to make less confident predictions about the future.