einfach
simple / easy
Not complicated, easy to understand or do.
Die Anleitung war sehr einfach zu befolgen.
The instructions were very easy to follow.

simple / just
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simple / easy
Not complicated, easy to understand or do.
Die Anleitung war sehr einfach zu befolgen.
The instructions were very easy to follow.

The word is made up of 'ein' (one) and '-fach'. '-fach' expresses multiplication (as in 'zweifach' — twofold). So 'einfach' literally means “single-fold” or “not multiple,” which explains the basic meaning “uncomplicated.”
Diese Aufgabe ist nicht einfach, sondern dreifach so schwer.
This task is not simple, but three times as difficult.
As an adjective, 'einfach' describes a noun and is declined. As a particle, it is invariable and gives a statement a certain nuance, e.g., 'mach es doch einfach' (just do it).
Das ist eine einfache Lösung für ein komplexes Problem.
That is a simple solution to a complex problem.
As a particle, 'einfach' has no meaning of its own, but rather emphasizes or simplifies a statement. It can express impatience, a simple fact, or a suggestion that needs no further explanation.
Ich habe die Prüfung einfach nicht bestanden.
I simply didn't pass the exam.
In the context of travel, tickets, and train tickets, 'einfach' always means “one-way only.” The opposite is 'hin und zurück' (round trip/there and back).
Einmal nach Berlin, einfach, zweite Klasse bitte.
One to Berlin, one-way, second class please.
only
'nur' and 'einfach' can seem similar in some sentences because both can make a statement smaller or more direct. But 'nur' limits something: it means that there is no more than a certain thing, or that only one thing applies. 'einfach' as a particle more often emphasizes that you do something without a long explanation, without a detour, or as a clear solution. Learners often confuse them because both can sometimes feel like 'just'. If you are limiting an amount or choice, use 'nur'. If you want to say that someone simply does something or does not need a complicated solution, use 'einfach'.

Wenn der Termin nicht passt, rufe ich einfach morgen noch einmal an.
If the appointment doesn't work, I'll just call again tomorrow.
Nach dem Einkauf habe ich nur noch fünf Euro in der Tasche.
After shopping, I only have five euros left in my pocket.