notwendig
necessary
Absolutely required or indispensable for a particular purpose.
Für diese Reparatur sind spezielle Werkzeuge notwendig.
Special tools are necessary for this repair.

necessary
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necessary
Absolutely required or indispensable for a particular purpose.
Für diese Reparatur sind spezielle Werkzeuge notwendig.
Special tools are necessary for this repair.

The word is made up of 'Not' (need/distress) and the old verb 'wenden' (in the sense of 'to avert'). Something is 'notwendig' (necessary) when it is needed to avert an emergency or achieve a goal.
Ein Visum ist notwendig, um in dieses Land zu reisen.
A visa is necessary to travel to this country.
'Notwendig' (necessary) is very often used in sentences with 'es ist ...' (it is ...). This is frequently followed either by an infinitive clause with 'zu' (to) or by a subordinate clause with 'dass' (that).
Es ist notwendig, die Regeln zu kennen.
It is necessary to know the rules.
When something is required for a purpose, 'notwendig' (necessary) is often combined with nouns such as 'Maßnahmen' (measures), 'Schritte' (steps), 'Informationen' (information), or 'Unterlagen' (documents).
Bitte bringen Sie alle notwendigen Unterlagen zum Termin mit.
Please bring all the necessary documents to the appointment.
important
'wichtig' and 'notwendig' can appear in similar situations, but they do not mean the same thing. 'wichtig' means that something has great importance. 'notwendig' means that something is absolutely needed or cannot be avoided. Learners confuse the words because something necessary is often also important. But not everything important is necessary. Good preparation can be important, but a valid passport can be necessary for a trip.

Vor der Reise ist ein gültiger Ausweis notwendig.
Before the trip, a valid ID is necessary.
Bei der Bewerbung ist ein freundliches Anschreiben wichtig.
In a job application, a friendly cover letter is important.