"Whose is this garden?" should be accepted as an alternative (shouldn't it?). (This garden belongs to whom?)
Does Ukrainian invert word order for questions?
No. Unless it is an open question (e.g. with what, who, why e.t.c.), there is no way to tell a question from a statement, apart from the presence of a question mark (when reading), or tone of voice (when speaking).
Why is a "й" used here instead of a "я" at the end of whose?
Because "сад" is a masculine noun. I suggest you to read Tips&Notes.
I figured it out soon after I posted that, thanks though!
I don't understand how to pronounce Ч. Also, is the sound of Я and й the same?
Ч sounds like the 'ch' in chair, because it looks like an upside down chair :)
Oooh! That's good xD It'll definitely help me remember the sound! xD
Basically, я = йа. When we write phonetic transcriptions for our words that's exactly what we do: моя [мойа]
If this is like Russian than I believe that it is pronounced as a ch like in chicken, and the Я sounds like ya, as in yall in the southern dialect. and the Й is just a y sound I believe.
what is the difference between chyy and chyya? is it just masculine vs feminine?
Yes, чий is masculine, чия is feminine.
how it pronounce to Chyy
See the question by Butterflys_Dust and the corresponding answers.