Qualities

heavy

Source:*tęžьkъProto-Slaviccertain

Only Polish keeps the nasal of *tęž-; elsewhere it became я (Russian), ě (Czech) or e (BCS).

I knowI'm learning

English

heavy

Polish

ciężki

ˈtɕɛ̃ʂkʲi

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In all five languages

BCS script

Latin under Cyrillic

Russian
тяжёлыйtjažëlyj
tʲɪˈʐolɨjinherited
The same root with another suffix; тя́жкий also exists, meaning 'grave, severe'.
Ukrainian
важки́йvažkyj
ʋaʒˈkɪjdifferent word · Ukrainian вага́ 'weight', from German Waage via Polish waga
The cognate тяжки́й also exists, mostly for 'hard, grievous'.
Polish
ciężki
ˈtɕɛ̃ʂkʲiinherited
Czech
těžký
ˈcɛʃkiːinherited
Bosnian-Croatian-Serbian
težakтежак
ˈteʒakinherited
English
heavy