schlicht
plain / simple
Simple in form, without decoration or elaborate details; unobtrusive and understated in style.
Sie trägt am liebsten schlichte, einfarbige Kleidung.
She prefers to wear plain, solid-colored clothing.

simple / plain
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plain / simple
Simple in form, without decoration or elaborate details; unobtrusive and understated in style.
Sie trägt am liebsten schlichte, einfarbige Kleidung.
She prefers to wear plain, solid-colored clothing.

The meaning of 'schlicht' (simple/plain) changes depending on its function in the sentence. As an adjective before a noun, it describes something as simple and unobtrusive. As an unchanged adverb before another word (often an adjective or verb), it acts as an intensifier and means 'simply' or 'downright'.
Das Restaurant ist für sein schlichtes, aber elegantes Ambiente bekannt.
The restaurant is known for its simple but elegant atmosphere.
When 'schlicht' (plain/simple) describes a simple style, it is often associated with design, art, and fashion. Typical combinations are 'schlichte Eleganz' (simple elegance), 'ein schlichtes Design' (a simple design), 'schlichte Möbel' (plain furniture), or 'ein schlichtes Kleid' (a plain dress).
Mir gefällt das schlichte Design der skandinavischen Möbel.
I like the simple design of the Scandinavian furniture.
In its function as an intensifying adverb, 'schlicht' (simply/downright) often comes before adjectives that express an extreme or absolute quality. Common combinations are 'schlicht unmöglich' (simply impossible), 'schlicht falsch' (simply wrong), or 'schlicht beeindruckend' (simply impressive).
Es ist schlicht unmöglich, diese Aufgabe an einem Tag zu erledigen.
It is simply impossible to complete this task in one day.
In the meaning 'simple and understandable', 'schlicht' (simple) is often used with abstract nouns. A very common fixed expression is 'die schlichte Wahrheit' (the simple truth).
Manchmal ist die schlichte Wahrheit schwer zu akzeptieren.
Sometimes the simple truth is hard to accept.
simple / just
'einfach' and 'schlicht' can both mean that something is not elaborate or not complicated. 'einfach' is very general: it can refer to an easy task or to a simple style. 'schlicht' is especially suitable for form, clothing, furniture, or design and means: without many details or decoration. They can be confused when talking about style. A dress can be 'einfach' or 'schlicht'. But for a math problem, you would more naturally say 'einfach', not 'schlicht'.

Im Laden haben wir einen schlichten Tisch ohne Verzierungen gekauft.
At the store, we bought a plain table without decorations.
Heute sucht das Team eine einfache Lösung, die nicht viel Geld kostet.
Today the team is looking for a simple solution that does not cost much money.