台
dai
台 is a machine on its stand
The picture The character 台 means 'stand'. Picture a machine resting whole on its stand, holding down a space of its own. It fits in your palm: 個. It sits in a spot on the floor or the desk: 台.
Where the picture shows up in 台
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the one you ride
空港でレンタカーを一台借りました。
kūkō de rentakā o ichidai karimashita
I rented a car at the airport.
Opening the door of the rental car in the airport car park and climbing in.
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the one on the desk
事務所にパソコンが三台あります。
jimusho ni pasokon ga sandai arimasu
There are three computers in the office.
Three office desks with a computer sitting on each, one to a desk.
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the one you bring home
洗濯機を一台買い替えました。
sentakuki o ichidai kaikaemashita
We replaced the washing machine.
The washing machine lowered off the truck and pushed into its spot.
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the one parked there
入り口に自転車が二台止めてあります。
iriguchi ni jitensha ga nidai tomete arimasu
There are two bikes parked at the entrance.
Two bicycles standing side by side in the rack at the entrance.
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asking how many units
この部屋にエアコンは何台ありますか。
kono heya ni eakon wa nandai arimasu ka
How many air conditioners are in this room?
Looking up at the ceiling and pointing at each air-conditioning unit in turn.
Wheeled things and machines take 台, but aircraft take 機 (き) and ships take 隻 (せき).
All of it in one picture
The rental car is climbed into, the computer settles on a desk, the washing machine is pushed into its spot, the bikes stand in the rack, the air conditioner holds a place on the wall. In all five, something whole comes to rest somewhere and claims that space. 台 doesn't count things because they're machines — it counts them because they sit on a stand and hold a spot. That's why a bicycle, with no motor at all, is still 一台.