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to buy

Source:*kupitiProto-Slaviccertain

Perfectives, the everyday forms. Czech koupit shows the change ú → ou.

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English

to buy

Polish

kupić

ˈkupʲitɕ

CognateSame source — the sound laws connect them.

In all five languages

BCS script

Latin under Cyrillic

Russian
купи́тьkupitʹ
kʊˈpʲitʲinherited
Ukrainian
купи́тиkupyty
kuˈpɪtɪinherited
Polish
kupić
ˈkupʲitɕinherited
Czech
koupit
ˈkou̯pɪtinherited
Bosnian-Croatian-Serbian
kupitiкупити
ˈkupitiinherited
English
to buy
English shares this source: cheap — Proto-Slavic took *kupiti from Germanic (Gothic kaupōn 'to trade'), whose root, from Latin caupo 'trader', also gave English cheap.