Learn French: Le client dont le neveu est égoïste ne va pas payer. - The client whose nephew is selfish is not going to pay.

Yes, dont translates to "whose" here. However, unlike English, where we simply say "whose nephew", French requires a definite article (le, la, or les) before the possessed noun. Therefore, "whose nephew" translates to dont le neveu.

Could I say Le client dont son neveu est égoïste?

No, because dont already indicates possession. Adding the possessive adjective son ("his") makes it redundant, like saying "The client whose his nephew" in English. You must use the definite article (le), not a possessive word, after dont.

Why does the negative ne ... pas go around va instead of payer?

This sentence uses the near future structure, combining a conjugated verb (va) with an infinitive (payer). In French, the ne ... pas always hugs the conjugated verb. The infinitive stays outside the negative sandwich, giving us ne va pas payer.

How do we know if égoïste is masculine or feminine here?