Introduction:

Among is used when something or someone is part of a group.
Underneath is used when something or an idea is lower than something else. It is physically under.
Beneath is used in the same fashion as underneath. It can also be used as a figurative below.
Between is used when something or someone is found between two things.

Structure:

among / underneath / beneath / between / are adjectives.
This group of adjectives functions as typical adjectives.

Examples:

We found the lost document among others. (Notice others is plural)
It was underneath the book. (You can say book or books) It us to be physically below.
It was beneath the desk. (You can also use desks)
The account manager was between appointments. (appointments must be plural)