ŝtupo

Here the seam is real, and it is the textbook example. A ŝtupo is one step. Ŝtuparo is the staircase, and it is ŝtupo plus -ar-, the collective suffix that gathers a set of things into the thing they add up to. Arbo gives arbaro (a forest), vorto gives vortaro (a dictionary), homo gives homaro (humanity), and ŝtupo gives the stairs. If you have ever wondered why Esperanto learners talk about the affixes the way they do, this is the reason: you already knew ŝtuparo at rank 732, and this page just told you the root it was built from, at rank 2,366.

A useful third member. Ŝtupetaro is a ladder: a little staircase, ŝtup- plus the diminutive -et- plus -ar-, which you can carry. Notice the order the affixes stack in - ŝtupetaro is a collection of small steps, not a small collection of steps - and that the language lets you read that off the word.

Careful with number. English says the stairs and treats them as plural; Esperanto says la ŝtuparo, singular, because the staircase is one object. Use ŝtupo when you mean an individual step - la tria ŝtupo krakas, the third step creaks - and ŝtuparo for the whole flight.

The figurative sense comes free. A ŝtupo is also a stage or a rung: la unua ŝtupo de la kariero, the first step of a career, or ŝtupo post ŝtupo, step by step. Keep it apart from paŝo, which is the step you take with your piedo when walking. A staircase has ŝtupoj; a walk has paŝoj.