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As for the term appearance, why do you guys use the plural form for 1), but not 2)?

1) Don’t let appearances fool you.

2) You shouldn’t judge people by their appearance.

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teacherJapanAs for the term appearance, why do you guys use the plural form for 1), but not 2)?

The two forms mean different things.

teacherJapan1) Don’t let appearances fool you.

"Appearances" are the superficial aspects of things, visual or otherwise.

teacherJapan2) You shouldn’t judge people by their appearance.

A person's appearance is how they look.