нельзя

From Old East Slavic нельзѣ (nelĭzě), нельзѧ (nelĭzę), нельза (nelĭza), from Proto-Slavic *ne / *ně + *lьga (“possibility”) (possibly influenced by its dative *lьdzě), likely related to *lьgъkъ (whence лёгкий (ljóxkij, “light, easy”)).

Compare Ukrainian не́льга (nélʹha), нільга́ (nilʹhá), нельзя́ (nelʹzjá), Belarusian няльга́ (njalʹhá), нільга́ (nilʹhá), не́льга (njélʹha), Old Church Slavonic льзѣ (lĭzě, “possible, possibly”), Slovak nelza, nelzä, neľzä, Old Polish nielza.

• IPA^(key): [nʲɪlʲˈzʲa]

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нельзя́ • (nelʹzjá) (impersonal, invariable)

1. it is impossible (perfective aspect), one cannot, you can't

   нельзя́ ему́ + perfective infinitive ― nelʹzjá jemú + perfective infinitive ― it is impossible for him to (perfective infinitive)

2. it is prohibited, it is forbidden, it is not allowed, one must not

   здесь нельзя́ кури́ть ― zdesʹ nelʹzjá kurítʹ ― it is not allowed to smoke here

Нельзя́ is the negative of мо́жно (móžno). Instead of saying не мо́жно, нельзя́ should be always used.

• невозмо́жно (nevozmóžno)
• запрещено́ (zapreščenó)

• мо́жно (móžno)