
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.

just / simply
Used as a particle to emphasize or simplify a statement, or to present it as having no alternative. It often means “plain and simple” or “that's just how it is.”
Warum hast du das getan? – Einfach so.
Why did you do that? — Just because.

plain / modest
Describes something that is plain, modest, and without special decoration or luxury.
Er führt ein sehr einfaches Leben auf dem Land.
He leads a very simple life in the countryside.

one-way
Refers to a ticket or flight that is valid only for the outward journey and not for the return journey.
Ich hätte gern eine Fahrkarte nach Hamburg, einfach bitte.
I'd like a ticket to Hamburg, one-way please.
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
In a casual request, both nur and einfach can serve as encouragement. Nur is more likely to remove hesitation or concerns, while einfach presents the action as an uncomplicated, obvious solution. Depending on the intended nuance, both can appear in the same sentence.

Ruf mich einfach an, wenn du noch Fragen hast.
Simply call me if you have any more questions.
Ruf mich nur an, wenn du noch Fragen hast.
Just call me if you have any more questions.