unusal parts of speech

[Unregistered]

I'm trying to identify the parts of speech in this sentence.

Let's keep this just between us.

[Casiopea]

I'm trying to identify the parts of speech in this sentence.

Let's keep this just between us.

Let's = imperative verb form (fused; let + us)
us = (unfused, object of "let")
keep = infintive verb, bare, object of "Let's")
this = demonstrative pronoun; substantive in function
between us = prepositional phrase; function (see below)

If "keep between us" is idiomatic in nature, then it's a prepositional verb, and "between us" functions as an integral part of the verb "keep". If it's not idiomatic in nature, if "keep this" stands alone, if it's meaning doesn't change when "between us" is added, then it's a verb + adverbial phrase:

TEST
Let's keep this between us.
1. This (information) is to be kept in this file. (adverbial; stored)
2. This (information) is to be kept between us. (prepositional phrase; object; kept secret)

There is a difference in meaning between 1. and 2. In 2. "keep between" means, keep a secret.

Hope that helps.