Lately, I've been reading about demonstrative pronouns and adjectives. I'm fairly comfortable with hic and ille, but I'm not too sure about is. How is is weaker than the other demonstratives? When would you use is instead of the others?
I don't have a source for this answer, as it's based on my intuition from reading Latin texts, but here's my sense of the difference between is and hic/iste/ille.
Hic/iste/ille are strongly deictic: that is, they have a kind of "pointing finger" meaning. They often point out objects in space (here, near you, over there); but they can also point out things in metaphorical or "discourse space" -- this thing I'm about to say, that thing you're saying, that thing someone said a long time ago.
Is, on the other hand, doesn't really point out a referent in any distinctive way; it just means "the one we're talking about". It's a way of referring to something that's strongly "discourse-active", i.e. currently activated as a topic of discussion in the speaker's and hearer's minds. This is why, used by itself, is gets translated as "he" (which presupposes that the listener knows who "he" is), while hic etc. get translated as "this/that one" (which points out a referent out of a set of possibilities).
Though you didn't ask about them, the spatial adverbs show the same distinction. There's ibi, eo, inde "in/to/from the place we're talking about", vs. hic, huc, hinc "in/to/from this place here", istic, istuc, istinc "in/to/from that place of yours", illic, illuc, illinc "in/to/from that place there".