deprimieren
to depress
To make someone feel down, sad, or discouraged.
Das ständige schlechte Wetter deprimiert mich total.
The constant bad weather totally depresses me.

to depress
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to depress
To make someone feel down, sad, or discouraged.
Das ständige schlechte Wetter deprimiert mich total.
The constant bad weather totally depresses me.

The verb 'deprimieren' (to depress) is used actively, with the cause of the sadness as the subject and the person who becomes sad as the accusative object. The pattern is: Something depresses someone.
Die ständige Kritik deprimiert die Mitarbeiter.
The constant criticism depresses the employees.
Very often, the past participle 'deprimiert' (depressed) is used as an adjective with the verb 'sein' (to be) to describe a person's state. It answers the question 'Wie fühlst du dich?' (How do you feel?).
Nach der Absage war er tagelang deprimiert.
After the rejection, he was depressed for days.
The subject of the verb 'deprimieren' (to depress) is often abstract things or situations that trigger a negative mood. Common examples are 'das Wetter' (the weather), 'die Nachrichten' (the news), 'die Einsamkeit' (loneliness), or 'ein Misserfolg' (a failure).
Die Nachricht von der Werksschließung deprimierte die ganze Stadt.
The news of the factory closure depressed the whole town.
to knock down / suppress / be reflected
'niederschlagen' and 'deprimieren' can both have to do with a bad mood, but they are not used in the same way. 'deprimieren' means that something makes someone sad, discouraged, or hopeless. In the meanings listed here, 'niederschlagen' is mainly a word for violence, for example when someone is knocked to the ground. The words can be confused because there is also the adjective 'niedergeschlagen': a 'niedergeschlagen' person is sad or discouraged. For the verb in this emotional meaning, 'deprimieren' usually sounds clearer and more natural.

Das schlechte Prüfungsergebnis hat Tom sehr deprimiert.
The poor exam result really depressed Tom.
Die Polizei konnte den gewalttätigen Angreifer schnell niederschlagen.
The police were able to knock down the violent attacker quickly.