Why can't you say "Ja mam to jablko"? or "Ja mam jablko?"
The both of your translations should be recognized.
Why do you need ja? Isn't mam enough?
Translations without já also are accepted.
So the sentence structure is
To ____ mám já
Can I put anything in instead of jablko? I have cheese, I have bread, I have money?
TO jablko mám já. / TEN sýr mám já. / TY peníze mám já.
I get that "to" is neutral,
"ten" is masculine,
"ta" is feminine.
But then what is "ty"? I've never seen it in the course used this way. Shouldn't it be "to"/"ten"/"ta"? (I don't know what gender peníze is)
TY is plural (masc. inanimate and feminine), you will meet it soon.
Why in this way?
Placing já at the end of the sentence emphasizes that YOU are the one who has the apple, not someone else.
Why can't it be "Mám to jablko"?
It certainly can, it is accepted. You have to use the report button to report missing translations.