Let's = imperative verb form (fused; let + us)Originally Posted by Unregistered
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:
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.