Como é que se diz isto em Inglês (EUA)? Most of her stationery are hand-her-downs from her older sister.

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This is a tough question even for a native speaker haha! Since the rest of the sentence takes the plural form, "stationary" can take the pural form, which would be "stationaries." This isn't a commonly used word but in this context it makes the most sense.

The correct word is "hand-me-downs" regardless of what they are or who they came from. (Example: All I have to wear are hand-me-downs from my older brother.)

So the correct sentence would be: Most of her stationaries are hand-me-downs from her older sister. You could also write: Most of her stationary was handed down by her older sister. Same meaning and stationary can take the singular form, I think.

Hard question! Hopefully other native speakers will chime in and help too.