why here plural (przydatne) and singular, (it is)? together
as usually the hint is misleading.
As with all adjectives:
nom. sing. neuter = nom. plural not masc pers= acc. sing neuter=acc. plural not masc pers
here it is nominative singular neuter, as it describes "to"
Why not "Is this useful?"
It's an accepted answer, it should have worked.