How do languages borrow words, and what is a calque?
Borrowing is taking a word from another language. A calque — or loan translation — is borrowing the structure of a phrase while translating its parts.
Straightforward loanwords. English has taken in extraordinary numbers: piano and opera from Italian, café and genre from French, kindergarten and rucksack from German, sofa and algebra from Arabic, tsunami and emoji from Japanese, bungalow and shampoo from Hindi.
What happens to borrowed words:
Adaptation to the borrowing language's sounds, since speakers substitute the nearest available phonemes.
Meaning narrowing. A borrowed word usually takes a narrower sense than in the source. French chef means any boss or head; English borrowed it for one kind. German Handy means a mobile phone, which is not English at all.
Nativisation — acquiring normal inflection and losing the original plural. Stadium mostly takes stadiums now.
A calque translates the parts. English skyscraper was calqued into French as gratte-ciel, Spanish rascacielos and German Wolkenkratzer — each translating the components rather than importing the word.
Running the other way, English borrowed calques without most speakers realising:
Loanword itself is a calque of German Lehnwort.
Flea market from French marché aux puces.
Brainwashing from Chinese xǐnǎo.
Beer garden from Biergarten, and superman from Übermensch.
Why languages borrow. New concepts and objects arriving with a name attached; prestige, where the source language carries status — which is why English took so much from French after 1066 and why English technology vocabulary is now borrowed worldwide; and gaps where no native term exists.
Related phenomena: semantic loans, where an existing word gains a new sense from another language; false loans such as Japanese salaryman, which look English and are not; and re-borrowing, where a word returns altered — English animation went to Japan and returned as anime.