Sedere vs. sedersi
While both Italian words sedere and sedersi revolve around the act of sitting, their usage is distinctly different. The core distinction lies in whether the verb is reflexive (acting on oneself) or transitive (acting on someone or something else).
Sedere
B1This is the transitive verb form, meaning to seat someone or something. It requires a direct object—the person or thing being seated. It's used far less frequently in this form than its reflexive counterpart. Additionally, sedere is also a masculine noun meaning bottom, rear, or buttocks.
L'usciere deve sedere gli invitati in prima fila.
(The usher must seat the guests in the front row.)
La mamma ha seduto il bambino sul seggiolone.
(The mother seated the child in the high chair.)
Siedi la bambola sul letto, per favore.
(Seat the doll on the bed, please.)
È scivolato e ha battuto il sedere per terra.
(He slipped and hit his bottom on the ground.)
Sedersi
A1This is the reflexive verb form and is the standard, most common way to say to sit down. The action of sitting is performed by the subject on themselves. The reflexive pronoun (mi, ti, si, ci, vi, si) must be used, and it changes depending on the subject.
Prego, si sieda su questa poltrona.
(Please, sit down in this armchair.)
Appena sono entrato, mi sono seduto sul divano.
(As soon as I came in, I sat on the sofa.)
Perché non vi sedete e bevete qualcosa?
(Why don't you all sit down and have a drink?)
Ci siederemo fuori per goderci il sole.
(We will sit outside to enjoy the sun.)
Siediti e raccontami tutto.
(Sit down and tell me everything.)
Il gatto ama sedersi sulla finestra.
(The cat loves to sit on the windowsill.)
Summary
In essence, use sedersi when someone sits themselves down, which is the vast majority of cases (e.g., I sit down is Io mi siedo). Use the non-reflexive sedere only when you are actively seating another person or object (e.g., I seat the guest), or when you are referring to the noun for bottom.