"This is my son."

Translation: To jest mój syn.

[Ktwu]

Why is it 'syn' and not 'synem'? Does the fact that 'to' is present change the casing, even though 'jest' normally requires the instrumental form?

[immery]

Yes. "To" before "jest" changes the cases from instrumental to nominative. It is also the word that does not require "jest" so "to mój syn" is also correct.

https://www.duolingo.com/skill/pl/Defining- read the part about "to"

[mat420942]

Why can't i use the word Ten?

[Trofaste]

"To jest" is fixed, "to" doesn't change with gender in this construction.

[Ash473779]

Why does it it use the 'mojego' pronoun here?

[Jellei]

Sorry, what's the question? There's no 'mojego' here, that's a very simple sentence...