Why do we use that'll in question 1?

"Must" contradicts "probably", so it's like saying a big little dog, or a quick slow car. That **will** probably be... is a prediction. Normal English. And often abbreviated to "That'll".

Tbh I would use any of these, I'd probably say "that'd" myself.

I'm not a native English speaker, can we say "That could probably be the cat?" or is it redundant since I've already used "could" to also say "probably"?

"That must probably" doesn't work. "That'll" means "That will", so "That will probably be the cat" though it does sound a bit clunky and unnatural for a native speaker, they're more likely to say "that's probably the cat"

"Must" implies a level of certainty, but "probably" expresses doubt. Having both of them in the sentence simultaneously creates confusion. "That will probably be the cat" doesn't.

Because probably usually goes with will in grammar. I will probably go. ✅ I must probably go. ✖️

'must' is a statement of certainty. While you might occasionally hear a native speaker do it, as far as the grammar is concerned, they're incompatible.

having the “must” and “probably” next to each other just sounds wrong. im not sure if theres a grammar rule for it. just sounds wrong

must is a modal verb which is basically absolute certainty or obligation - "That must be the cat" is saying "there is no other possibility." Will in this case which implies a state of certainty but is not absolute. So while "That *must* be the cat" and "That *will* be the cat" function as the same basic concept, 'will be' allows us to guess at (speculate) our accuracy. "That will <speculation> be the cat". "That will *probably* be the cat" "That will *most likely not* be the cat" and so on. Must is just the most certain, so you can't speculate.

Note: I'm American, and this test is not American English. I would say "That would probably be the cat." if I had to put something in that spot, but I think it would be more natural to just say "That's probably the cat." I don't think I'd ever say "That'll" in this case, though I don't think it's necessarily wrong. I would say "That'll" in a situation where you're definitely going to find out, such as if you hear a knock at the door or a telephone ring. "That'll probably be my mother." Because we're going to go find out, and "will" implies that mother will be what appears when we find out. I would never say "That must" here because "must" + "probably" are contradictory. "Must" means that it can be nothing other than the cat, while "probably" means there is a chance that it is not the cat.

I'd choose that option because "must probably" is instant nonsense. Those two words conflict with each other. That said, you won't see the construction of "that will" or "that'll" much in everyday English. It's grammatically sound but it's not common outside a few phrases. I would use that phrase "That'll probably be..." if I didn't hear the noise but I still had *expectations* (which might be why we use the future tense "will") of what the other person heard. Just my opinion - that phrase is almost archaic sounding, like it belongs in a Western, but it does make perfect sense to a native speaker.

how does will/probably go together tho? i'd say "that'd probably..." sounds more natural, and im a native speaker

You can say "that must be the cat." You can say "that's probably the cat." You can't say "that must probably be the cat." Must and probably are contradictory; one indicates certainty and the other, an uncertainty. That'll means *that will.* It's perfectly correct to say that.

"Must" implies something definite but "probably" suggests a degree of ambiguity, so the two together are contradictory. The "will" in "that'll" isn't as strongly definite, so the logical contradiction isn't there. It's complicated though, as a sentence like "that must be the cat, surely?" is fine even though ambiguity is suggested. It works because a definite statement is then questioned rather than the statement contradicting itself. In colloquial speech people in the UK would just say "must be the cat" with the object and verb being implied. "That'll be the cat" suggests that the speaker definitely knows (or at least believes they know) that the cat made the noise. "Must be the cat" could suggest by the tone of voice they are still slightly uncertain, even though it uses a more definite verb.

Am I the only one who thinks "that'll" sounds off and should be "that would"?

You can say "That must be the cat" or "That's probably the cat" but "That must probably" does not work. Probably and Must conflict with each other... think of it as 75% certainty and 100% certainty, they both can't be true; You can't say them in the same sentence about the same thing. "That will probably be the cat" does work however

It doesn’t make sense whether it’s ‘that’ll’ or ‘that.’

This is not how Americans speak English.  "Must" and "probably" contradict each other.  You'd either say "that's probably the cat" if you aren't sure, or "that must be the cat" if you are sure. It makes more sense to say it in the present tense though.  "That'll be the cat" sounds like the wrong tense to me.

I don't think either would be correct. As others had said, "That must probably" is nonsensical because must and probably contradict each other, which is probably why it's incorrect. But saying "That will" also seems incorrect in this context because it suggests that the cat is going to be the source of the noise that they just heard. Like, they heard a noise, the cat didn't make it, but it will. That doesn't make sense either.

I feel like, maybe in England more so than America. We just wouldnt say that. It doesn't sound right over here

We can use "will" to predict the truth that we do not yet know for certain. It denotes an expectation, and *can* be used to imply an assumption that it would be awkward to contradict: "You'll be wanting to kiss your Auntie Margaret, who's been waiting to see what a fine young man you're growing up to be; your friend here will probably prefer to leave his muddy boots in the porch." Often, however, it's just a guessing game.

“Must” implies certainty, so it clashes with the adverb “probably.” But to me, “will” also doesn’t sound good either. We’re talking about a sound we just heard, and we’re talking about what the source of the sound WILL be rather than IS? It’s better to say “that’s probably the cat.”

American: I would say "did you hear that noise?" "It's probably just the cat"

I would probably say "that's", just sounds more fluent to me

"That'll" doesn't seem to be the best answer but it's more appropriate than "that must". Personally I'd say "that is" or "it is"

Honestly, I question your test. Neither is a great answer, but between the two, I would pick "that must probably." In reality, the correct answer would be "That was probably the cat."

The only time that "must probably" as a phrase gets used is when one is doing comedy. It isn't normal conversation. It's akin to saying, "50 percent of the time, it works every time." This statement has a 50% of the time, which means not always, then follows up with a statement that means always.

"That'll be the cat" is more of a British way to say it. You don't hear that exact expression much in North American English.

I read this in an English grandfather’s voice while smoking his pipe and ruffling his newspaper “Well well then that’ll probably be the cat, cheerio then dear I’m off to the haberdasher” lmao