
bereit sein
to be ready
To be prepared and willing to do something or for something to happen.
Ich bin bereit, wir können losfahren.
I'm ready; we can set off.
ready
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to be ready
To be prepared and willing to do something or for something to happen.
Ich bin bereit, wir können losfahren.
I'm ready; we can set off.

to be willing to
To show readiness or willingness to do something specific, often in the sense of making a concession or effort.
Sie war zu allem bereit, um ihre Ziele zu erreichen.
She was willing to do anything to achieve her goals.

to stand by
To be in a waiting position in order to act immediately if needed.
Die Rettungskräfte halten sich für den Einsatz bereit.
The rescue teams are standing by for the operation.
The adjective 'bereit' (ready) belongs to the word family of 'bereiten' (to prepare) and 'Vorbereitung' (preparation). All of these words have to do with preparing for an action or event.
Nach langer Vorbereitung war die Mannschaft bereit für das Finale.
After a long period of preparation, the team was ready for the final.
'Bereit' (ready) is almost always used predicatively with the verb 'sein' (to be), meaning after the noun. Using it directly before a noun, for example 'der bereite Läufer', is very rare and sounds unnatural.
Die Sportler sind bereit für den Wettkampf.
The athletes are ready for the competition.
To express what someone is willing to do, you use a construction with 'zu'. Either a noun in the dative follows ('bereit zu einem Kompromiss' — willing to make a compromise), or an infinitive clause follows ('bereit, einen Kompromiss zu machen' — willing to make a compromise).
Der Politiker ist zu einem Gespräch mit der Opposition bereit.
The politician is willing to have a conversation with the opposition.
In the meaning 'to wait for deployment/action,' the reflexive verb 'sich bereithalten' (to stand by) is used. The reflexive pronoun (sich, mich, dich, etc.) must always be included and match the person.
Die Feuerwehr hält sich für mögliche Waldbrände bereit.
The fire department is standing by for possible forest fires.
clear
Klar can mean in nautical or military language that something is prepared for an upcoming action. Bereit is the general word and can additionally express a person's willingness. Outside such special contexts, bereit is usually the better choice.

Die Besatzung ist bereit zum Ablegen.
The crew is ready to depart.
Die Besatzung macht das Boot klar zum Ablegen.
The crew prepares the boat for departure.