Questions & Answers about Hans enorma hund är mycket snabb.
Why is the adjective enorma in its -a form, but hund doesn't have a definite ending like hunden?
This is the special rule for possessives like Hans (his). When you use a possessive word, the adjective following it must take the definite form (adding an -a, making it enorma), but the noun itself stays in the basic indefinite form (hund).
Since the dog is definite because of Hans, why isn't snabb also written as snabba?
Adjectives only get that special definite -a ending when they are modifying the noun directly and come right before it. Because snabb comes after the verb är, it just follows standard agreement rules. Since hund is a singular en-word, snabb stays in its basic form.
Could I use sin instead of hans here since it also means "his"?
No, because hans enorma hund is the subject of the sentence. You only use sin, sitt, or sina when "his" refers back to the subject of the sentence (like in "He loves his dog"). When the dog itself is the subject, you must use hans.
Does the word mycket change form if the noun is an ett-word or plural?