了
le
了 is not finished, it is a line you just crossed
The picture A line chalked on the floor, and the moment your foot lands on the far side. Before the line and after the line are not the same. 了 crossed the line and left things changed; 着 never crossed it and leaves the picture standing as it was.
Where the picture shows up in 了
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the action crossed over
我已经交了报告。
wǒ yǐjīng jiāo le bàogào
I have already handed in the report.
The report leaves your hands, lands on his desk, and you turn away empty-handed.
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the weather crossed over
下雪了，路上小心。
xià xuě le, lù shang xiǎoxīn
It is snowing now, watch your step.
The first flakes settle on dry asphalt and the road turns white.
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your mind crossed over
我不想去了。
wǒ bù xiǎng qù le
I do not feel like going anymore.
You reach for your shoes, then set them back down by the door.
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the amount crossed over
太贵了，能便宜点吗？
tài guì le, néng piányi diǎn ma
That is too expensive, can you come down a bit?
The needle climbs with the price tag and slides past the red line you drew beforehand.
太…了 has nothing to do with the past. It says the price stepped over the line you could live with.
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crossed and still going
我在这儿住了三年了。
wǒ zài zhèr zhù le sān nián le
I have been living here three years now.
The threshold you stepped over three years ago is still behind you, and you are still standing in the room.
All of it in one picture
The report leaving your hands is the action crossing over; dry ground turning white is the weather crossing. Shoes going back down is your mind crossing, the needle passing the red mark is the amount crossing, and standing in a room you entered three years ago is a crossing that keeps running. Five pictures, one single step; all that changes is where the line was drawn. So when 了 turns up, do not ask what finished, ask what line was crossed.