aufstellen
to set up
To put an object in a place, usually in an upright position.
Wir müssen das Zelt aufstellen, bevor es dunkel wird.
We have to set up the tent before it gets dark.

to set up / nominate
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to set up
To put an object in a place, usually in an upright position.
Wir müssen das Zelt aufstellen, bevor es dunkel wird.
We have to set up the tent before it gets dark.

'Aufstellen' is a separable verb. The prefix 'auf-' goes at the end in a main clause. In subordinate clauses and in the infinitive with 'zu', the verb stays together.
Er stellt die Stühle im Garten auf.
He sets up the chairs in the garden.
In this meaning, the verb is used for physical objects that you put somewhere. Typical examples are: 'ein Zelt aufstellen' (to set up a tent), 'Möbel aufstellen' (to arrange furniture), 'ein Schild aufstellen' (to put up a sign), or 'eine Leiter aufstellen' (to set up a ladder).
Die Stadt hat neue Bänke im Park aufgestellt.
The city set up new benches in the park.
This meaning refers to creating abstract things. Common combinations are: 'Regeln aufstellen' (to establish rules), 'eine Liste aufstellen' (to draw up a list), 'eine Theorie aufstellen' (to put forward a theory), or 'einen neuen Rekord aufstellen' (to set a new record).
Der Wissenschaftler hat eine neue Hypothese aufgestellt.
The scientist put forward a new hypothesis.
When it is about nominating people, 'aufstellen' (to nominate) is almost always found in a political context. Signal words are 'Kandidat' (candidate), 'Partei' (party), and 'Wahl' (election).
Welchen Kandidaten stellt Ihre Partei für die Wahl auf?
Which candidate is your party nominating for the election?
The meaning 'position' is very often used in sports when a coach arranges the players for a game. A typical object is 'die Mannschaft' (the team) or 'das Team' (the team).
Der Trainer hat die Mannschaft sehr offensiv aufgestellt.
The coach lined up the team very offensively.
to put / to set
'stellen' and 'aufstellen' can both mean that you put an object in a place, often in an upright position. 'stellen' is more general and fits many everyday situations: you put a cup, a bag, or a chair somewhere. 'aufstellen' sounds more purposeful and is often used when something is set up for a purpose or put somewhere visibly, for example a sign, a tent, or a Christmas tree. Learners may confuse the words because both have to do with a standing position. 'aufstellen' places more emphasis on preparing or setting something up.

Nach dem Essen stellt er im Garten einen kleinen Tisch auf.
After dinner, he sets up a small table in the garden.
Am Morgen stellt der Mitarbeiter die Pflanzen ans Fenster.
In the morning, the employee puts the plants by the window.