"They are strange!"
Because it does not work here. Here you are refering to some they and give them some adjective, there is no other object. Compare with: Those are strange people. To jsou zvláštní lidé.
Sorry, but no. "Jsou to zvláštní.... what?" It is kind of as if you said in English, "they are the strange"
I understand, I think. So you would not say "Jsou to zvlastni hrady"? but you could say "Jsou ty vlastni hrady"? or "Ci jsou to zvlastni hrady?" All these to's and ty's are hard to process.
Suppose I insert a noun instead of an adjective. For example: Jsou to zvířata. Is that correct, meaning 'they are animals'?
Are "Jsou oni zvláštní" or "Jsou ony zvláštní" possible alternative translations? Probably not, because it was rejected. Though, I would like to know why.. thx.
"Jsou oni zvláštní?" is a word order of a question. If it was to be a statement, it would sound too poetic.
"Jsou zvláštní." is a neutral statement, if you want to emphasize that THEY are weird/strange, you put it in the initial position "Oni jsou zvláštní." or to emphasize the word "zvláštní", you say "Zvláštní jsou oni."
What's the difference between oni, ony and ona ? I can't find an explanation anywhere... and when I asked a Czech friend, she said that only "oni " is possible, so now I'm even more confused
"Oni, ony and ona" all translate as "they". The difference between them is in grammatical gender. "Oni" is used for masculine animate nouns; "ony" for both masculine inanimate and feminine nouns; and "ona" for neuter nouns. On top of that "ona" can also mean "she" in singular, so watch out for the verb in Czech (if it's in singular or plural) to see which one is used.
When you have a group of people and it contains at least 1 male (ie. masculine animate), use "oni". When the group contains only women (ie. feminine) and children (ie. neuter), use "ony". The same applies also when constructing the past tense.