Lass uns einfach so tun, als würden wir uns nicht kennen!

English Translation

Let’s just pretend as if we don’t know each other.

why is the verb not “Lassen” since the subject is “wir”?

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Because it is an imperative.

But it is indeed a difficult example.

“Lass …” is the informal imperative singular, that means the speaker is talking to only one other person and (s)he is on a first name base with this person.
If (s)he would be speaking to more than one person, it would be “Lasst …”
If (s)he would to speaking formaly it would be “Lassen sie …” no matter, if adressing one or more than one person.

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