
heulen
to cry
To cry loudly and with tears, often in an uncontrolled or childlike way.
Das kleine Kind begann zu heulen, als es sein Spielzeug verlor.
The little child began to cry when it lost its toy.
to cry / howl
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to cry
To cry loudly and with tears, often in an uncontrolled or childlike way.
Das kleine Kind begann zu heulen, als es sein Spielzeug verlor.
The little child began to cry when it lost its toy.

to howl
To make the long, plaintive sound typical of certain animals such as wolves or dogs.
In der Ferne hörte man die Wölfe im Wald heulen.
In the distance, you could hear the wolves howling in the forest.

to howl
To produce a loud, high-pitched, and sustained sound, as sirens, wind, or engines do.
Der Motor des Rennwagens heulte auf, als der Fahrer beschleunigte.
The race car's engine howled as the driver accelerated.
The subject of the sentence reveals the meaning: if a person 'heult' (cries), they are crying. If an animal, e.g. a wolf, 'heult' (howls), it makes a long sound. If an object, e.g. a siren, 'heult' (howls), it produces a loud sound.
Er heulte vor Glück, als er die Nachricht hörte.
He cried with joy when he heard the news.
When it refers to people, 'heulen' (to cry) is a very colloquial and emotional word for 'weinen' (to cry). It describes loud, often uncontrolled crying.
Hör auf zu heulen, es ist doch nichts Schlimmes passiert!
Stop crying—nothing terrible happened!
A fixed and very vivid expression is 'Rotz und Wasser heulen'. It means that someone is crying extremely hard and without restraint.
Nach der traurigen Nachricht hat sie Rotz und Wasser geheult.
After the sad news, she cried her eyes out.
to cry
Weinen and heulen can both express crying with tears. Heulen, however, sounds clearly more colloquial and often refers to especially loud or uncontrolled crying; it can also be derogatory. Use weinen for a neutral description and heulen when this stronger or coarser nuance is intended.

Das Kind heulte nach dem verlorenen Spiel laut in seinem Zimmer.
The child loudly bawled in his room after losing the game.
Das Kind weinte nach dem verlorenen Spiel leise in seinem Zimmer.
The child quietly cried in his room after losing the game.