In general, word groups (words that evolved from the same original word and meaning) are used as mnemonic devices that allow us to better remember word meanings, which allows language to be learned faster. Like, if you know what "basis" means, you already have a clue as to what base, bases, basic, the basics, etc. mean. You're at least pointed in the right direction and not thinking they mean something about food or outer space.
Unfortunately, word groupings - and related devices like prefixes and suffixes - can lead to words that sound alike but have nothing to do with one another. That's the case with etymology and entomology. If you stutter, it'd be hard to tell which one you're saying.
The backend of the word is part of the problem - logy is a suffix (ending to a word) that means "the study of" or "the science of". It's Latin and used most commonly with science and academic words.
Entomology is the study of insects, while etymology is the study of words, specifically how words evolve. (In other words, it's a bit ironic that I screw up the meaning of this one, but irony itself could easily be another entry in this series, so I won't go there.)
To be honest, I believe the problem with these two words is that I learned them both aurally first, by hearing other people say them, having no idea how they were spelled, and my brain somehow heard them as the same word. (Hey, if chair can mean "a thing you sit on" and "the person leading a committee," then why can't this fancy word I don't know have multiple meanings?) And when I learned them properly, my mind never had a moment where it caught it's own mistake, so the two remained cojoined in my mind.
In fact, when I teach and I want to say etymology (I teach college English, so I don't teach about entomology that often), I have to think about it really hard before I open my mouth. And I still probably screw it up about half the time. Maybe someday, my mind will remember on its own, but I definitely googled these words again just now, just to make sure I didn't screw it up again before I made this post.
What pairs of words do you routinely mix up?