"behave like us" vs "behave like we (do)"

Question

Consider a sentence:

They behave like us

Why do we use "us" here? Should it be subjective pronoun "we" as it is a subject of implied verb "do", i.e.

They behave like we (do)

Which one is correct and why?

Answer 1

You've got it backwards. First, you look at what people actually say, then you come up with an analysis. If people actually said *they behave like we, then the ellipsis analysis would be reasonable. But since we don't say that, the ellipsis analysis must be wrong.

Instead, like must be introducing a noun phrase, not a verb phrase, and so the accusative us is the natural choice. This is perfectly standard and has been for hundreds of years. Of course, it can introduce a verb phrase as well:

 1. They behave like us. (standard, introducing noun phrase)

 2. They behave like we do. (standard, introducing verb phrase)

Historically, some prescriptivists have taught people that option 1 is incorrect, despite the fact that it's standard English. But even those who reject option 1 above would not omit do:

 3. *They behave like we. (non-standard omission of do)

So if you want to say we, make sure you say do as well.

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In this answer, * marks a sentence that is not standard English.