schlicht
plain
Simple in form and design, without elaborate decorations or ornamentation.
Sie bevorzugt schlichte Möbel im skandinavischen Design.
She prefers plain furniture in Scandinavian design.

plain / simple
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plain
Simple in form and design, without elaborate decorations or ornamentation.
Sie bevorzugt schlichte Möbel im skandinavischen Design.
She prefers plain furniture in Scandinavian design.

In the meaning of 'simply designed', 'schlicht' (plain/simple) is often used to describe design, clothing, or interior furnishings. Typical combinations are 'schlichte Eleganz' (simple elegance), 'ein schlichtes Kleid' (a plain dress), or 'schlichtes Design' (plain design).
Für das Büro trägt er meist ein schlichtes, weißes Hemd.
For the office, he usually wears a plain white shirt.
When a person is described as 'schlicht' (simple), it is usually meant positively. It expresses that someone is down-to-earth, natural, and not arrogant or complicated.
Er ist ein Mann von schlichtem Gemüt und freut sich über die kleinen Dinge im Leben.
He is a man of simple character and takes pleasure in the little things in life.
The expression 'schlicht und ergreifend' (plain and simple) is a fixed expression. You use it to emphasize a simple, undeniable fact and put an end to further discussion.
Warum ich nicht mitkomme? Ich habe schlicht und ergreifend keine Lust.
Why am I not coming along? I plain and simple don't feel like it.
simple / just
'einfach' and 'schlicht' can both describe something without much decoration, luxury, or special detail. 'einfach' is broader and can also mean that something is easy or not complicated. 'schlicht' is especially suitable for clothing, furniture, shapes, or design. It emphasizes a clear, calm style without ornamentation. The words can be confused with things like dresses or rooms. Practically speaking: if you mean that something is easy, use 'einfach'. If you mean a style without decoration, 'schlicht' is more precise.

Der schlichte Tisch passt gut in die kleine Küche.
The plain table fits well in the small kitchen.
Frau Meier kauft ein einfaches Regal für den Flur.
Frau Meier buys a plain shelf for the hallway.