二/两
èr / liǎng
Chinese has two number twos
The picture Read the 2 printed on a sign and it comes out 二; put two of something in your palm and count them, and it becomes 两. When the two things counted are people, 两个人 shortens to 俩 — but 俩 already contains the measure word, so 俩个人 is wrong.
Where the picture shows up in 二/两
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the 2 on a label
我住十二楼，请按一下。
wǒ zhù shí'èr lóu, qǐng àn yíxià
I'm on the twelfth floor — could you press it?
Your fingertip on the lift button marked 12, saying the number out loud
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two of a thing
老板，来两杯冰美式。
lǎobǎn, lái liǎng bēi bīng měishì
Two iced americanos, please.
Two cups set side by side on the counter while you hold up two fingers
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two units of time
我等了两个小时，他还没来。
wǒ děng le liǎng ge xiǎoshí, tā hái méi lái
I waited two hours and he still hasn't turned up.
Sitting in a café watching the clock, folding down a finger for each hour that passes
Two o'clock is 两点, never 二点.
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the 2 that ranks
前面第二个路口右转。
qiánmiàn dì-èr ge lùkǒu yòu zhuǎn
Turn right at the second corner ahead.
The first turning slipping past, and a little ② tag hanging at the mouth of the next one
After 第 it is always 二. 第两个 does not exist.
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two before a big unit
这个包两千八，能便宜点儿吗？
zhège bāo liǎng qiān bā, néng piányi diǎnr ma
This bag is 2,800 — could you come down a little?
Two bundles of thousand-yuan notes lifted off the counter in front of the price tag
Use 两 before 千 and 万; before 百 the standard form is 二百.
All of it in one picture
Reading a floor number aloud gives you 二; ordering two cups gives you 两; two hours stacking up gives you 两 as well. Ranking things puts you back on 二 with 第二, and two bundles of notes make 两千. The test never changes: is this a number you are reading out, or two things you are picking up and counting?