klug
clever
Intelligent and quick to understand; able to think, learn, and solve problems well.
Sie ist eine sehr kluge Schülerin mit ausgezeichneten Noten.
She is a very clever student with excellent grades.

clever / wise
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clever
Intelligent and quick to understand; able to think, learn, and solve problems well.
Sie ist eine sehr kluge Schülerin mit ausgezeichneten Noten.
She is a very clever student with excellent grades.

When 'klug' (clever) refers to a person, it means intelligent. When it refers to an action, an idea, or a decision, it means sensible or wise.
Mein Großvater ist ein sehr kluger Mann.
My grandfather is a very clever man.
A very common construction is 'es ist klug, etwas zu tun' (it is wise to do something). It is used to evaluate an action as sensible.
Es ist klug, sich vor einer langen Reise gut zu informieren.
It is wise to get well informed before a long trip.
The expression 'aus jemandem/etwas nicht klug werden' means that one cannot understand a person or thing. It is always formed with the preposition 'aus' and the dative.
Ich lese den Text schon zum dritten Mal, aber ich werde daraus einfach nicht klug.
I'm reading the text for the third time, but I still simply can't make sense of it.
stupid / unfortunate
'dumm' and 'klug' are opposites when talking about thinking or decisions. 'dumm' means that something shows little sense or has not been thought through well. 'klug' means that someone understands well, thinks carefully, or makes sensible decisions. For people, 'klug' describes the ability to think and learn. For actions, 'klug' fits especially when a decision is sensible and forward-looking.

Er handelt klug, weil er vor dem Kauf erst die Preise vergleicht.
He is acting wisely because he compares the prices before buying.
Vor der Reise war es dumm, den Pass nicht noch einmal zu kontrollieren.
Before the trip, it was stupid not to check the passport again.