"This marriage"

Translation: To małżeństwo

[Leodarelli]

Ń has sound of Ni but softly?

[Jellei]

They're almost the same, "ni" kinda has as a short 'i' (compare "koń" vs "koni").

[Kulink]

Why to instead of te or tego?

[tadjanow]

Gender neuter noun, singular and nominative case means that one should use 'to'. 'Te' would be used with plural 'te małżeństwa' and 'tego' with genitive case 'tego małżeństwa'

See the declension table at:

https://en.wiktionary.org/wiki/ten#Polish

[lunatix17]

Why not "Ten slub" ?

[Jellei]

That's "This wedding".