sich setzen
to sit down
To take a sitting position.
Nach dem langen Spaziergang wollte ich mich nur noch setzen.
After the long walk, I just wanted to sit down.

to set / put
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to sit down
To take a sitting position.
Nach dem langen Spaziergang wollte ich mich nur noch setzen.
After the long walk, I just wanted to sit down.

It is important to distinguish between 'sich setzen' (to sit down) and 'etwas setzen' (to put/place something). 'Sich setzen' is reflexive and means that you sit yourself down. 'Etwas setzen' is transitive and means that you place an object somewhere.
Ich setze mich auf den Stuhl.
I sit down on the chair.
'Setzen' is a very productive base verb in German. If you know it, you can understand many other important verbs more easily, for example 'übersetzen' (to translate), 'einsetzen' (to use / insert), 'fortsetzen' (to continue), or 'ersetzen' (to replace).
Der Dolmetscher muss den Satz sofort übersetzen.
The interpreter has to translate the sentence immediately.
When you place something, the verb 'setzen' needs an accusative object (what you are placing) and an indication of direction (where you are placing it). The question is always 'Wohin?' (where to?), so a two-way preposition is used with the accusative.
Er setzte seinen Namen unter den Vertrag.
He put his name under the contract.
In the meaning 'vertrauen auf' (to rely on) or 'wetten auf' (to bet on), 'setzen' is always used with the preposition 'auf' and the accusative.
Ich setze auf den Sieg meiner Lieblingsmannschaft.
I am betting on my favorite team winning.
In the meaning 'festlegen' (to establish/set), 'setzen' is often combined with abstract nouns. Common phrases are 'Grenzen setzen' (to set boundaries), 'Prioritäten setzen' (to set priorities), 'ein Zeichen setzen' (to send a signal), or 'Maßstäbe setzen' (to set standards).
Es ist wichtig, klare Prioritäten für das Projekt zu setzen.
It is important to set clear priorities for the project.
In a gardening context, 'setzen' is used for plants that you put into the soil as a whole or as a tuber/bulb. Typical combinations are 'Kartoffeln setzen' (to plant potatoes), 'Zwiebeln setzen' (to plant onions/bulbs), or 'einen jungen Baum setzen' (to plant a young tree).
Wir haben gestern die Tulpenzwiebeln für das nächste Frühjahr gesetzt.
Yesterday we planted the tulip bulbs for next spring.
The expression 'etwas aufs Spiel setzen' (to put something at risk) is a fixed unit. The thing being risked appears as an accusative object before 'aufs Spiel'.
Er hat seinen Job aufs Spiel gesetzt, um die Wahrheit zu sagen.
He put his job at risk in order to tell the truth.
to put / to set
'stellen' and 'setzen' can both mean that you put something in a place. The important difference lies in the type of object and its position. 'stellen' is used especially when something stands upright, for example a bottle, a vase, or a chair. 'setzen' can more generally mean 'to place', but it often sounds less concrete or fits fixed situations. Learners confuse the words because both are connected with a place. A practical tip: if an object is standing afterward, 'stellen' is usually more natural.

Im Formular setzt du ein Kreuz neben die richtige Antwort.
In the form, you put a cross next to the correct answer.
Bitte stell die Flasche auf den Tisch, damit sie nicht umfällt.
Please put the bottle on the table so it doesn't fall over.