上
shàng
It begins the moment your foot lands: 上
The picture Lifting one foot off the ground and planting it on some platform. 上 steps onto the platform and so is inside it; 下 lifts off that platform and so is out of it.
Where the picture shows up in 上
-
foot on the floor of the bus
快上车，要开了。
kuài shàngchē, yào kāi le
Get in, we're about to leave.
A foot leaving the pavement, finding the bus step, and landing on the floor inside
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step onto the working day
我每天八点上班。
wǒ měitiān bā diǎn shàngbān
I start work at eight every day.
Crossing the office threshold in the morning and setting your feet down on the office floor
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step onto the lesson
我下午还要上课。
wǒ xiàwǔ hái yào shàngkè
I still have class this afternoon.
Pushing the classroom door open and putting your feet down at your desk
上课 works for the teacher and the student alike — whoever steps onto that platform.
-
step onto the network
这里可以上网吗？
zhèlǐ kěyǐ shàngwǎng ma
Can I get online here?
A phone screen lighting up as the signal bars fill in and you stand on the network
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come up onto the table
菜马上就上。
cài mǎshàng jiù shàng
The food will be right out.
A plate leaving the kitchen, passing through a waiter's hands, and landing on the table
All of it in one picture
上 is one foot stepping onto a platform. Step onto the floor of the bus and you have boarded; step onto the working day and you are at work; step into the lesson and you are in class. Step onto the network and you are online; when the plate steps up onto the table, the food has arrived. So wherever 上 turns up, something that was outside has planted a foot on that platform and is now inside it.