台
tái
The counter for things that run once switched on: 台
The picture One unit placed on its base that starts turning under its own power the moment you switch it on. 台 counts one unit that runs on its stand; 辆 counts one unit that rolls on wheels.
Where the picture shows up in 台
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one unit sitting on a desk
我们办公室有五台电脑。
wǒmen bàngōngshì yǒu wǔ tái diànnǎo
There are five computers in our office.
One monitor on each desk, power lights on down the row.
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one unit that has stopped turning
这台机器坏了。
zhè tái jīqì huài le
This machine is broken.
A machine on the shop floor gone silent with only a red lamp left on.
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one unit mounted in place
房间里装了一台空调。
fángjiān lǐ zhuāng le yì tái kōngtiáo
We had an air conditioner put in the room.
A unit fixed high on the wall, air just starting to come out of it.
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one run from start to finish
医生今天做了三台手术。
yīshēng jīntiān zuò le sān tái shǒushù
The doctor did three surgeries today.
The theatre lamp going on, one case finishing and the next one wheeled in.
An operation treated as one 'run' of a machine. 场 and 次 also occur.
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one unit holding its spot at home
他家买了一台新冰箱。
tā jiā mǎi le yì tái xīn bīngxiāng
They bought a new fridge.
A fridge taking up its corner of the kitchen, humming low all night.
All of it in one picture
The computers sitting on the desks are 台, the machine that has stopped is 台, the air conditioner bolted to the wall and blowing is 台. The fridge holding down its corner of the kitchen all night is 台, and so is a surgery started and run to the end in theatre. What 台 counts is not size but 'a unit that, once in place, runs by itself'. That is why things with no power going into them rarely take 台.