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.

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 need
'brauchen' and 'fehlen' can both show that something is necessary or that there is not enough of something. The difference is mainly in the point of view and in the grammar. With 'brauchen', the focus is on the person or thing that needs something: 'Ich brauche Geld' (I need money). With 'fehlen', the thing that is not there is often the subject: 'Mir fehlt Geld' (I am missing money / I don't have enough money). Learners easily confuse the words because both talk about a lack. As a guide: 'brauchen' says what someone needs; 'fehlen' says what is not available.

In meinem Antrag fehlt noch die Unterschrift.
The signature is still missing from my application.
Für die Reise brauchen wir noch einen gültigen Pass.
For the trip, we still need a valid passport.