Usage Note
In appears in Spanish text almost exclusively in loanword phrases and fixed Latin or English borrowings: in situ, in vitro, in vivo, show-in, stand-in. It is not a productive Castilian preposition — the standard equivalents are en (location/state) and dentro de (inside). Learners should use en in their own writing and recognise in only in these fixed international terms.
Examples
"El experimento se realizó in vitro."
Natural Translation
The experiment was carried out in vitro.
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