
fehlen
to be absent
Not to be present even though one is expected; to be absent.
Heute fehlen drei Schüler in der Klasse.
Today three students are absent from class.
to be absent / to be missing
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to be absent
Not to be present even though one is expected; to be absent.
Heute fehlen drei Schüler in der Klasse.
Today three students are absent from class.

to be missed
To be missed by someone; to leave an emotional gap. The person or thing that is missed is the subject of the sentence.
Seit du weg bist, fehlst du mir jeden Tag.
Since you've been gone, I miss you every day.

to be missing
Not to be present, or not present in sufficient quantity; to be lacking.
Uns fehlt das Geld für eine große Reise.
We're missing the money for a big trip.

What's wrong?
A typical question a doctor asks a patient to ask about their complaints or symptoms.
Der Arzt fragte den Patienten: 'Guten Tag, was fehlt Ihnen denn?'
The doctor asked the patient: 'Good day, what's the matter?'

to miss
In sports or a game: to miss a target or make an error.
Der Stürmer hat beim Elfmeter gefehlt und den Ball über das Tor geschossen.
The striker missed on the penalty and shot the ball over the goal.
The grammar helps distinguish the meanings. If a person in the dative (e.g. 'mir', 'dir') appears with 'fehlen' (to be absent / to be missing), it is usually about a feeling of missing someone or a lack of something. Without a dative person, it often means 'to be absent'.
Mir fehlt die Energie, heute noch Sport zu machen.
I'm lacking the energy to do sports today.
To express that you miss someone, the sentence structure is different from English. The person who is missed is the subject (nominative), and the person who has the feeling is in the dative.
Meine beste Freundin fehlt mir sehr.
I miss my best friend very much.
When something is not present, the thing that is missing is in the nominative (subject). The person who is missing something is in the dative.
Mir fehlt der Mut, ihm die Wahrheit zu sagen.
I'm lacking the courage to tell him the truth.
In the meaning 'abwesend sein' (to be absent), 'fehlen' is often used in contexts like school, university, or work. Typical formulations are 'im Unterricht fehlen' (to be absent from class) or 'bei der Besprechung fehlen' (to be absent from the meeting).
Er hat letzte Woche wegen Krankheit bei der Arbeit gefehlt.
He was absent from work last week because of illness.
The question 'Was fehlt Ihnen?' (What's wrong? / What seems to be the problem?) is a standard phrase that doctors or pharmacists use to ask about a person's health problems or symptoms. In informal conversation, people say 'Was fehlt dir?' (What's wrong?).
Der Notarzt fragte die verletzte Frau: 'Sagen Sie mir, was Ihnen fehlt.'
The emergency doctor asked the injured woman: 'Tell me what's wrong.'
To express a lack, you can also use the impersonal structure 'es fehlt an + dative' (there is a lack of). This form is somewhat more formal.
In vielen Regionen fehlt es an sauberem Trinkwasser.
In many regions, there is a lack of clean drinking water.
to miss
Fehlen and vermissen can express the same emotional absence, but from different perspectives. With fehlen, the missed person is the subject and the person who feels the absence is in the dative. With vermissen, the person who feels the absence is the subject and the missed person is the accusative object.

Seit Anna im Ausland lebt, fehlt sie ihrem Bruder sehr.
Anna has lived abroad ever since, and her brother misses her terribly.
Seit Anna im Ausland lebt, vermisst ihr Bruder sie sehr.
Ever since Anna moved abroad, her brother has missed her terribly.