I’ve already written about general pronoun choice, but the we/us dilemma is worth tackling separately. We is a nominative pronoun, which means that it is used as a subject, and us is an objective pronoun, which means that it is used as an object, but I doubt that clears up the confusion that occurs when we or us is next to a noun that describes it.
We/Us humans are composed of 60 chemical elements.
The noun humans describes the we/us pronoun. This kind of construction often poses a problem, but the solution is generally simple: Just drop the noun that describes the pronoun, and the correct pronoun choice becomes obvious.
In our example sentence, we would drop humans, which would give us We/Us are composed of 60 chemical elements. The correct choice is, of course, we, so when we put humans back into the sentence, it reads We humans are composed of 60 chemical elements.
There’s nobody here but we/us cowards.
Just drop the noun cowards, which describes the we/us pronoun, and the correct pronoun choice becomes apparent. Is it There’s nobody here but we or There’s nobody here but us? Obviously, the correct one is us, so when we put the noun cowards back in, the full sentence becomes There’s nobody here but us cowards.
We/Us students get too much homework. (We)
The company takes advantage of we/us employees. (us)
Note: You’ll notice that in each of these cases, we is, indeed, the subject of a verb and us, indeed, is an object.