unglücklich
unhappy
Experiencing a state of sadness, grief, or emotional suffering.
Er ist schon seit Wochen unglücklich, weil er seinen Job verloren hat.
He has been unhappy for weeks because he lost his job.

unhappy / unfortunate
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unhappy
Experiencing a state of sadness, grief, or emotional suffering.
Er ist schon seit Wochen unglücklich, weil er seinen Job verloren hat.
He has been unhappy for weeks because he lost his job.

The word is the opposite of 'glücklich' (happy). The prefix 'un-' is frequently used in German to express negation or the opposite.
Sie war mit der Entscheidung sichtlich unglücklich.
She was visibly unhappy with the decision.
The meaning changes depending on the sentence structure. If it describes a person's feeling ('sie ist unglücklich' — she is unhappy), it means 'sad'. If it describes an action ('sie ist unglücklich gefallen' — she fell unfortunately), it means 'through bad luck'. If it stands before a noun ('eine unglückliche Wahl' — an unfortunate choice), it means 'inappropriate'.
Er ist unglücklich in seine beste Freundin verliebt.
He is unhappily in love with his best friend.
If you want to name the reason for the sadness, you often use the preposition 'über' (about/over) followed by the accusative.
Die Mannschaft war unglücklich über die knappe Niederlage.
The team was unhappy about the narrow defeat.
In the meaning 'inappropriate' or 'clumsy', the word is often combined with nouns like 'Wahl' (choice), 'Formulierung' (wording/formulation), 'Entscheidung' (decision), or 'Zufall' (coincidence).
Es war eine unglückliche Entscheidung, das Projekt abzubrechen.
It was an unfortunate decision to cancel the project.
happy / lucky
'glücklich' and 'unglücklich' are opposites in several meanings. For people, 'glücklich' means happiness and satisfaction, while 'unglücklich' means sadness or grief. For situations, 'glücklich' can mean that something turns out well or luckily, while 'unglücklich' points to bad luck, unfavorable circumstances, or an unsuitable choice. They can be confused because both can describe not only feelings, but also outcomes or decisions. So pay attention to the context: Is it about a person, a situation, or an action?

Die Verkäuferin war unglücklich, weil die Lieferung wieder zu spät kam.
The saleswoman was unhappy because the delivery was late again.
Wir sind glücklich über die neue Wohnung.
We are happy about the new apartment.