bellen
to bark
The typical loud sound a dog makes, often as a warning, when excited, or while playing.
Der Hund des Nachbarn bellt jedes Mal, wenn der Postbote kommt.
The neighbor's dog barks every time the mail carrier comes.

to bark
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to bark
The typical loud sound a dog makes, often as a warning, when excited, or while playing.
Der Hund des Nachbarn bellt jedes Mal, wenn der Postbote kommt.
The neighbor's dog barks every time the mail carrier comes.

The meaning of the verb becomes clear from the subject. If the subject is a dog (or a similar animal), the literal meaning is intended. If the subject is a human being, it is the figurative, negative meaning.
Mein Hund bellt nie, er ist sehr ruhig.
My dog never barks; he is very quiet.
There is a very well-known German proverb with this verb: 'Hunde, die bellen, beißen nicht' (dogs that bark don't bite). It means that someone who makes loud threats is often harmless.
Mach dir keine Sorgen wegen seiner wütenden E-Mail. Hunde, die bellen, beißen nicht.
Don't worry about his angry email. Dogs that bark don't bite.
When 'bellen' (to bark) is used for people, it is always strongly negative and derogatory. It describes a short, loud, and unfriendly call that is perceived as inhuman or aggressive.
Hör auf, mich so anzubellen! Sag es doch in einem normalen Ton.
Stop barking at me like that! Just say it in a normal tone.
to cry / howl
'heulen' and 'bellen' can both describe sounds made by dogs, but they are different sounds. 'bellen' is the typical short, loud sound of a dog, for example when it hears someone or wants to play. 'heulen' is a long, plaintive sound, like with wolves or sometimes dogs. Learners may confuse the words because both occur with animals. For the normal dog sound, you usually say 'bellen'. 'heulen' fits when the sound is drawn out and sad or sounds like a wolf's howl.

Vor der Tür bellt der Hund laut, als der Postbote kommt.
The dog barks loudly at the door when the mail carrier arrives.
In der Nacht heult der Hund, weil er allein im Hof ist.
At night, the dog howls because it is alone in the yard.