片
piàn
One thin flat slice: 片
The picture One slice shaved thin off a mass and laid out flat, so light it barely presses on your palm. 片 counts one thin flat slice; 块 counts one thick chunk broken off.
Where the picture shows up in 片
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one slice cut thin
我早上只吃了两片面包。
wǒ zǎoshang zhǐ chī le liǎng piàn miànbāo
I only had two slices of bread this morning.
Taking two thin slices out of the bread bag onto a plate.
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something pressed flat
饭后吃一片药。
fàn hòu chī yí piàn yào
Take one tablet after meals.
One flat round tablet resting on an open palm.
Capsules take 粒; flat pressed tablets take 片.
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one that fell of itself
地上落了一片叶子。
dì shang luò le yí piàn yèzi
A leaf has fallen on the ground.
A single leaf turning over as it settles flat on the ground.
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a surface laid out wide
窗外是一大片草地。
chuāng wài shì yí dà piàn cǎodì
Outside the window is a wide lawn.
Opening a window onto grass spread flat all the way to the horizon.
一大片 is common for a broad spread.
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one sheet covering everything
停电了，房间里一片黑。
tíngdiàn le, fángjiān lǐ yí piàn hēi
The power went out and the room is pitch black.
The lights dying and the whole room going dark as if a black sheet had dropped over it.
一片 is used when sound, light or colour fills a whole space.
All of it in one picture
The two slices of bread eaten at breakfast are 片, the tablet swallowed after the meal is 片, the leaf on the ground is 片. The lawn stretching outside the window is 一片, and so is the dark that fills the room when the power cuts. Take one thin slice and stretch it wide and you have a surface laid flat over everything. What 片 counts is 'whatever has given up its thickness and spread out flat'.