sitzen
to sit
To sit down or be seated with one's buttocks on a surface, for example on a chair or on the floor.
Wir saßen gemütlich auf dem Sofa und sahen einen Film.
We sat comfortably on the sofa and watched a film.

to sit
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to sit
To sit down or be seated with one's buttocks on a surface, for example on a chair or on the floor.
Wir saßen gemütlich auf dem Sofa und sahen einen Film.
We sat comfortably on the sofa and watched a film.

'Sitzen' (to sit) describes a position (Wo? + dative). The corresponding movement is expressed with the reflexive verb 'sich setzen' (to sit down) (Wohin? + accusative).
Er sitzt schon auf dem Stuhl.
He is already sitting on the chair.
The perfect tense of 'sitzen' (to sit) is formed with 'haben' in standard German. In southern Germany, Austria, and Switzerland, however, you also often hear the form with 'sein'.
Wir haben den ganzen Abend auf der Terrasse gesessen.
We sat on the terrace the whole evening.
Different prepositions are used depending on the type of seat: you sit 'auf' einem Stuhl (on a chair), 'in' einem Sessel (in an armchair), but 'an' einem Tisch (at a table).
Die Familie saß gemeinsam am Abendbrottisch.
The family sat together at the dinner table.
When 'sitzen' (to fit) means that clothing fits, the item of clothing is the subject of the sentence. A very common expression for this is 'sitzt wie angegossen' (fits like a glove).
Die Jacke ist perfekt, sie sitzt wie angegossen.
The jacket is perfect; it fits like a glove.
The meaning 'im Gefängnis sein' (to be in prison) is colloquial. You can indicate the reason with 'wegen' (+ genitive) or the duration with 'für' (+ accusative).
Er musste für zwei Jahre sitzen.
He had to be in prison for two years.
The expression 'sitzen bleiben' (to repeat a grade) is a fixed expression for having to repeat a school grade. It is often used in the passive.
Er ist in der achten Klasse sitzen geblieben.
He repeated eighth grade.
The construction 'auf etwas sitzen' (to be stuck with something) (with dative) means that you have a large amount of something and cannot sell it. The expression is often expanded to 'auf etwas sitzen bleiben' (to be left stuck with something).
Nach dem warmen Winter blieben die Händler auf ihren dicken Jacken sitzen.
After the warm winter, the retailers were left stuck with their thick jackets.
to stand
'stehen' and 'sitzen' both describe a body position. But they are opposites: with 'stehen', you are on your feet and your body is upright. With 'sitzen', you are sitting with your buttocks on a chair, a bench, or the floor. Learners may confuse the words because both often occur with places, for example on the bus, in the waiting room, or in the office. The key point is not the place, but the position of the body.

Im Bus sitzt sie neben ihrer Kollegin und liest eine Nachricht.
On the bus, she is sitting next to her colleague and reading a message.
Am Telefon steht mein Freund am Fenster und spricht leise.
On the phone, my friend is standing by the window and speaking quietly.