So if I want to say "your new garage", it should be "karaajik jadiidik", right?
Not quite. "Your new garage" would be "كَراجِك الجَديد", that is, you have to add a definite article "al-" before the adjective.
Thank you for your speedy reply! So may I summarize that whenever I want to use an adjective to modify a noun that is not indefinite, I have to add a definite article to the adjective?
Firstly, "your garage" is definite, i.e. if you add an indefinite noun to a pronoun, it becomes definite. Now, about the adjectives - I'll explain it by using a few examples. "A new garage" in Arabic would be "كَراج جَديد" (neither the noun, nor the adjective takes the definite article), "the new garage" would be "الكَراج الجَديد" (both, the noun and the adjective are preceded by the definite article), "the garage is new" would be "الكَراج جَديد" (only the noun takes the definite article). I hope, it's a little bit clearer now. If you still have any questions, feel free to ask.