对/给
duì / gěi
对 turns your face; 给 puts something in a hand
The picture 对 is your body turning to face someone. 给 is something leaving your hand and arriving in theirs. With 对, nothing changes hands.
Where the picture shows up in 对/给
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the way your face is turned
他对我笑了笑。
tā duì wǒ xiào le xiào
He gave me a little smile.
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what an effect lands on
抽烟对身体不好。
chōuyān duì shēntǐ bù hǎo
Smoking is bad for your health.
Good for, bad for, about — all of those are always 对.
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putting it in someone's hand
请给我一张收据。
qǐng gěi wǒ yì zhāng shōujù
Could I have a receipt, please.
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sending words across
我一会儿给你打电话。
wǒ yíhuìr gěi nǐ dǎ diànhuà
I'll give you a call in a bit.
Things that reach a person — 打电话, 发信息 — take 给. 对你打电话 is wrong.
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setting it aside as their share
我给你留了一份饭。
wǒ gěi nǐ liú le yí fèn fàn
I saved a plate of food for you.