to prevent / hold
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to prevent
To stop someone from doing a particular action or advise them against it.
Das schlechte Wetter sollte uns nicht davon abhalten, einen Spaziergang zu machen.
The bad weather shouldn't prevent us from going for a walk.
'Abhalten' (to prevent / to hold) is a separable verb. The prefix 'ab-' goes to the end of the sentence in a main clause. In subordinate clauses and in the infinitive with 'zu', the verb stays together.
Ich halte dich nicht von deiner Arbeit ab.
I'm not keeping you from your work.
You can recognize the meaning from the sentence structure: the construction 'jemanden von etwas abhalten' (to keep someone from something) means 'to prevent'. If the object is an event (meeting, course), it means 'to hold/conduct'. If the object is a disturbance (rain, cold), it means 'to keep out'.
Lass dich nicht von kleinen Problemen abhalten.
Don't let small problems stop you.
In the meaning 'hindern' (to prevent), 'abhalten' always follows a fixed pattern: jemanden (accusative) von etwas (dative) abhalten.
Die laute Musik hält mich vom Lernen ab.
The loud music keeps me from studying.
In the meaning 'durchführen' (to hold/conduct), 'abhalten' is typically used with organized events. Common combinations are: eine Sitzung abhalten (to hold a meeting), eine Konferenz abhalten (to hold a conference), Wahlen abhalten (to hold elections), or einen Gottesdienst abhalten (to hold a church service).
Die nächste Prüfung wird online abgehalten.
The next exam will be held online.
When 'abhalten' means 'fernhalten' (to keep away/out), the objects are usually unwanted influences. Typical ones are: Kälte abhalten (to keep out the cold), Regen abhalten (to keep out rain), Lärm abhalten (to keep out noise), or Insekten abhalten (to keep insects away).
Dicke Vorhänge können das Sonnenlicht gut abhalten.
Thick curtains can keep out sunlight well.
to prevent
'verhindern' and 'abhalten' can both express that someone does not do something. 'verhindern' is broader and can refer to people, events, or plans. 'abhalten' is used especially with people: someone or something keeps a person from doing an action. It often also suggests that someone is being warned, persuaded, or stopped by a reason. The words can be confusing because both describe not doing something. The typical structure is important: you 'verhindert etwas' (prevent something), but you 'hält jemanden von etwas ab' (keep someone from something).
Ihre Freunde halten sie davon ab, allein nach Hause zu gehen.
Her friends stop her from walking home alone.
Der starke Regen verhindert das Training am Abend.
The heavy rain prevents the training session in the evening.