YOU SAID:

I can always hear him sort of muttering and mumbling

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私はいつも彼がつぶやくようなつぶやきを聞くことができます

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I can always hear him muttering like

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私はいつも彼がつぶやくのを聞くことができます

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I can always hear him mutter

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私はいつも彼のつぶやきを聞くことができます

BACK INTO ENGLISH

i can always hear him tweet

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私はいつも彼のつぶやきを聞くことができます

BACK INTO ENGLISH

i can always hear him tweet