martelo

Here is a word that ought to have been built and was borrowed instead. A hammer is a tool, Esperanto has the tool suffix -il-, and a beginner could reasonably expect batilo - the thing you hit with. The language went to Latin for it anyway, and martel- is one indivisible root. Do not read mart-el-o; there is no mart-.

Why borrow when you can build? Because batilo is too vague to be useful. Anything you strike with is a batilo, from a cricket bat to a drumstick, and a language that names its tools purely by function ends up with one word for a dozen objects. Esperanto's usual practice is exactly this compromise: international roots for the specific things, home-grown affixes for the relations between them. You can still say batilo when you mean a beater, and everyone will follow you.

Everything downstream is regular. Marteli is to hammer, martelado is hammering as an activity, marteleto is a small one and martelego a sledgehammer. If you are naming the parts, the head is the kapo and the handle the tenilo - and there the -il- is genuine, because a handle really is the thing you hold with.