又
yòu
又 looks backward: the same footprint, pressed twice
The picture The same footprint pressed down a second time on ground you've already walked. 又 presses onto a footprint already there; 再 sets one down ahead, on ground not yet walked.
Where the picture shows up in 又
-
that footprint again
你怎么又迟到了？
nǐ zěn me yòu chí dào le
How are you late again?
The same stamp landing once more on the late-arrivals sheet, today's date under it.
Use 又 for what already happened, 再 for what hasn't. Say 明天再来, not 明天又来.
-
today on top of yesterday
昨天下雨，今天又下雨。
zuó tiān xià yǔ, jīn tiān yòu xià yǔ
It rained yesterday, and it's raining again today.
Rain falling on the same patch of ground before yesterday's wet marks have dried.
-
the same slip once more
我又忘了带钥匙。
wǒ yòu wàng le dài yào shi
I forgot my keys again.
Standing outside the door and picturing the keys still hanging on the hook inside.
-
the cycle comes back around
明天又是星期一了。
míng tiān yòu shì xīng qī yī le
Tomorrow is Monday again.
Turning one page of the calendar and finding a Monday square standing at the front again.
For a cycle that is already fixed, 又 is used even about the future.
All of it in one picture
又 looks backward, not forward. Another stamp on the late sheet is 'late again'; rain on marks that haven't dried is 'yesterday, and now today'; the keys on the hook seen from outside the door is 'forgot them again'. A fresh Monday at the front of the page is the cycle come back around. All four are the same footprint pressed onto ground already walked — which is why 又 can't carry something that hasn't happened yet.