As an adjective meaning simple, basic, or unadorned:
* Simples - This is the most common and general translation.
* Plano - This word specifically refers to something flat or level, but can also be used metaphorically for something simple or straightforward.
* Básico - This emphasizes the fundamental or essential nature of something.
* Sem graça - This means "without grace" and implies something lacking in beauty or sophistication.
* Sem enfeites - This literally means "without adornments" and emphasizes the lack of embellishment.
As a noun meaning a large area of flat land:
* Planície - This is the most common translation for "plain" as a noun.
* Campina - This word is used for a flat area with grass and few trees, often found in Brazil.
As an adjective meaning clear or obvious:
* Claro - This is the most common translation for "clear" or "obvious."
* Evidente - This means "evident" and emphasizes the undeniable nature of something.
As a noun meaning a type of fabric:
* Tecido simples - This means "simple fabric" and is a general translation.
* Sarja - This refers to a specific type of plain weave fabric.
As a verb meaning to make something smooth or level:
* Alisar - This means "to smooth" and can be used for both physical surfaces and abstract concepts.
* Nivelar - This means "to level" and refers specifically to making something flat or even.
The best translation for "plain" will depend on the specific context in which it is used.