Learn Icelandic: Jakkinn hans er stór og gamall.

In Icelandic, when you use a possessive pronoun like hans (his), the noun before it almost always takes the definite article (the "the" ending). The base word is jakki (a jacket), but here it becomes jakkinn (the jacket). So, you literally say "the jacket his".

Would the word hans change if we were talking about a feminine noun, like a book?

No, it wouldn't. While words like minn (my) change to match the gender of the object you own, hans is invariable. It is simply the genitive (possessive) form of hann (he), literally meaning "of him." The words hans (his), hennar (her), and þeirra (their) never change their form.

Can I put hans before the noun, like we do in English (His jacket)?

You can say hans jakki (without the "the" ending on the noun), but this usually places a strong emphasis on the fact that it is his jacket, not someone else's. In everyday conversational Icelandic, putting the possessor after the definite noun—jakkinn hans—is the standard and most natural way to say it.

Why are the adjectives stór and gamall in these exact forms?