be, look, seem

extremely, very | absolutely, completely
The dress was absolutely plain, but quite stunning.

fairly, quite, rather
The food was fairly plain, but well cooked.

plain blue, white, etc.
a plain white shirt

be, look

extremely, very
She looked very plain and dowdy.

rather
a rather plain woman

be, seem | become | make sth
They made it plain that they were against the idea.

very | absolutely, perfectly, quite
Within weeks, it became perfectly plain that we were in the grip of a tyrant.

increasingly | fairly, pretty, reasonably

to
It is all quite plain to me.

open
The horses galloped across the open plains.

rolling, sweeping
miles of rolling plain, made fertile by the river

great, unbroken, vast | flat | fertile
fertile plains suitable for farming

grassy | coastal

across a/the ~
Cattle move freely across the grassy plain.

in a/the ~
the Olduvai Gorge in the vast plain of Tanzania

on a/the ~
Nothing grew on the plain.