"This is my wife."
Translation:To jest moja żona.
The explanation is wrong. 'that woman' in polish would be 'tamta kobieta', while 'this woman' would be 'ta kobieta'. To refer to objects including people, masculine and femenine genders are not considered, to moja lampa, to moj przyjaciel, to moje okno, etc. TO kind of means this thing in here that is my... The same when someone wants to say in english 'It's me' you use IT to refer to I.
No. In a "This is Y" sentence, where "this" is a dummy pronoun serving as the subject of the sentence, Y is in Nominative.
If it was an "X is Y" sentence, like "Julia is my wife", Y would be in Instrumental: Julia jest moją żoną.
More info here: https://www.duolingo.com/comment/16373167