stöhnen
to groan / moan
To make a deep, inarticulate sound that expresses pain, great effort, or also pleasure.
Der Patient stöhnte leise vor Schmerz.
The patient groaned softly in pain.

to groan / moan
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to groan / moan
To make a deep, inarticulate sound that expresses pain, great effort, or also pleasure.
Der Patient stöhnte leise vor Schmerz.
The patient groaned softly in pain.

The two meanings can be distinguished well by the grammar. When someone complains about something, the preposition 'über' (+ accusative) is always used. When someone makes a sound because of pain or effort, the verb often stands alone or with 'vor' (+ dative).
Er stöhnte vor Schmerz, als der Arzt das Bein untersuchte.
He groaned in pain when the doctor examined his leg.
To express the cause of the physical groaning, you use the preposition 'vor' followed by the dative. Typical causes are pain, effort, or pleasure.
Der Läufer stöhnte vor Anstrengung, als er den Berg hinauflief.
The runner groaned with effort as he ran up the mountain.
When 'stöhnen' is used in the sense of 'sich beschweren' (to complain), it always takes the preposition 'über' with the accusative. This names the topic of the complaint.
Im Sommer stöhnen alle über die unerträgliche Hitze in der Stadt.
In summer, everyone complains about the unbearable heat in the city.
The use of 'stöhnen über' (to complain about) is rather colloquial and very vivid. It expresses that someone is complaining very intensely and audibly.
Mein Kollege stöhnt schon wieder über seine lange To-do-Liste.
My colleague is already complaining again about his long to-do list.
to complain / sue / lament
'klagen' and 'stöhnen' can both show that someone is suffering or is dissatisfied with a situation. 'klagen' is usually more verbal: you say in words what is bad or what you are suffering from. 'stöhnen' is first of all a sound, for example from pain or great effort. But it can also mean that someone talks loudly and plaintively about a burden. Learners may confuse the words because both occur with complaints. The important point is: 'stöhnen' emphasizes the audible sound or the great effort, while 'klagen' emphasizes the expressed complaint or suffering.

Beim Tragen der schweren Kisten stöhnt Max leise.
While carrying the heavy boxes, Max groans softly.
Nach dem langen Winter klagen viele Menschen über Müdigkeit.
After the long winter, many people complain about tiredness.