Bilingual speakers can swap languages mid-sentence without ever losing the thread

Fluent bilinguals routinely slide between two languages within a single conversation, sentence, or clause, dropping in a word or whole phrase from the other language and switching back seamlessly. Far from a sign of confusion, linguists have shown this code-switching follows its own consistent grammatical patterns, revealing what mental structure the two languages actually share.

— Pieter Muysken, Bilingual Speech: A Typology of Code-Mixing — Cambridge University Press, 2000
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