
fixieren
to fix
To attach something in a specific place or make it immobile.
Der Patient musste im Bett fixiert werden, um Stürze zu vermeiden.
The patient had to be fixed in the bed to prevent falls.
to fix / to stare at
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to fix
To attach something in a specific place or make it immobile.
Der Patient musste im Bett fixiert werden, um Stürze zu vermeiden.
The patient had to be fixed in the bed to prevent falls.

to stare at
To stare at someone or something with one's eyes; to direct one's gaze firmly at something.
Die Eule fixierte ihre Beute mit ihren großen Augen, bevor sie lautlos herabglitt.
The owl stared at its prey with its large eyes before gliding down silently.

to set
To set or definitively determine something, such as an appointment, price, or rule, in a binding way.
Können wir den Besprechungstermin für nächsten Dienstag um 10 Uhr fixieren?
Can we set the meeting date for next Tuesday at 10 o'clock?

to fixate on
To concentrate mentally or emotionally very strongly on a single person, thing, or idea, often in an obsessive way.
Er hat sich so auf die Prüfung fixiert, dass er alles andere vernachlässigt.
He has fixated so much on the exam that he is neglecting everything else.

to fix
In photography or chemistry: to make an image or a substance durable and resistant to further influences through chemical treatment.
Nach dem Entwickeln muss man das Foto in einem speziellen Bad fixieren.
After developing it, you have to fix the photo in a special bath.

to fix
In biology or medicine: to preserve a biological specimen, such as a tissue sample, in order to protect it from decay and prepare it for examination.
Die Gewebeprobe wurde direkt nach der Entnahme in Formalin fixiert.
The tissue sample was fixed in formalin immediately after removal.
Verbs ending in '-ieren' often come from Latin or French. They are always regular and form the past participle without 'ge-' ('fixiert').
Wir haben den Termin für die Reise jetzt endgültig fixiert.
We have now definitively set the date for the trip.
You can recognize the meaning of 'fixieren' (to fix / to stare at) from the object: if it is about a physical object, it means 'befestigen' (to attach). If it is about eyes or the gaze, it means 'anstarren' (to stare at). If it is about an appointment or price, it means 'festlegen' (to set).
Der Mechaniker fixierte das Rad mit vier Schrauben.
The mechanic fixed the wheel with four screws.
In the meaning 'befestigen' (to attach), the position is often given with the preposition 'an' (+ dative). Typical objects are shelves, signs, bandages, or even patients.
Das Bild muss sicher an der Wand fixiert werden.
The picture must be fixed securely to the wall.
In the meaning 'festlegen' (to set), 'fixieren' is often used in office and business language. It sounds more binding and final.
Wir müssen die Agenda für das Meeting noch heute fixieren.
We still have to set the agenda for the meeting today.
This meaning is always used reflexively and with the fixed preposition 'auf' + accusative: 'sich auf jemanden/etwas fixieren' (to fixate on someone/something). It describes a strong, often unhealthy concentration.
Du solltest dich nicht nur auf deine Fehler fixieren.
You shouldn't fixate only on your mistakes.
The meanings connected with photography, chemistry, or biology are purely technical. Without context words like 'Foto' (photo), 'Labor' (laboratory), or 'Gewebe' (tissue), this meaning is unlikely.
Im Labor lernen die Studenten, wie man ein Präparat für das Mikroskop fixiert.
In the laboratory, the students learn how to fix a specimen for the microscope.
to determine / notice / secure
Both words can mean that a movable part can no longer be moved. „Feststellen“ typically refers to locking an adjustable part in place, while „fixieren“ more generally refers to attaching or immobilizing something. Use „feststellen“ when there is a locking mechanism, and „fixieren“ for any kind of attachment.

Bitte fixiere den Sitz in dieser Position mit einem Gurt, damit er nicht verrutscht.
Please secure the seat in this position with a strap so that it does not move.
Bitte stelle den Sitz in dieser Position fest, damit er nicht verrutscht.
Please lock the seat in this position so that it does not move.