We must have accidentally switched the strollers.

[99bottles]

I have heard that, in most cases, the adverb goes after the first auxiliary verb. Is this true? If it's true, then why did I find so many examples with must have accidentally whereas I found no examples with must accidentally have? Isn't must the first auxiliary verb in that structure?

https://context.reverso.net/translation/english-french/must+have+accidentally

 

[Tarheel]

Well, I don't know about that "rule" but all those example sentences are quite natural.

(Since a native speaker usually knows what he knows implicitly and not explicitly it can be hard to explain things.)

Why is half of that in French?

 

[99bottles]

Well, I don't know about that "rule" but all those example sentences are quite natural.

(Since a native speaker usually knows what he knows implicitly and not explicitly it can be hard to explain things.)

Why is half of that in French?

So do you put the adverb before the main verb regardless of the number of auxiliary verbs?

 

[Tarheel]

It looks that way.
:-D

 

[Tdol]

When did most cases = all cases? You will come across cases where people say might possibly have happened, so it's your rule that needs refining.

 

[jutfrank]

I have heard that, in most cases, the adverb goes after the first auxiliary verb. Is this true?

That's not a good rule. I'm guessing that whoever told you that was thinking only about certain kinds of adverbs (adverbs of probability like possibly, certainly, definitely, etc.) since those words contribute to the modality of proximate modal auxiliaries. I don't know.

To speak in the most general way possible, the rule is that an adverb 'wants' to go directly next to the linguistic unit (word or phrase or sentence) that it modifies. In this case, it's quite clear: accidentally modifies the verb switched, so it goes directly next to it.