Learn Croatian: Njezin novi posao je mnogo bolji.

Both njezin and njen mean "her" and are completely interchangeable in Croatian. Njezin is slightly more formal or traditional, but you will hear both used frequently in everyday speech. They both agree with the masculine nominative noun posao.

Why do we use the adjective novi instead of nov here?

In Croatian, when an adjective comes after a possessive pronoun like njezin, it generally takes the definite form. That is why we use the definite novi instead of the indefinite nov.

How does the word bolji relate to the normal word for "good"?

Bolji is the comparative form of the adjective dobar (good), meaning "better". It is an irregular comparative, changing its root entirely just like "good" and "better" do in English. Here it is in the masculine singular form to agree with posao.

Can I use puno instead of mnogo to say "much better"?

Yes, absolutely. Puno and mnogo both mean "much" or "a lot" and can be used to strengthen a comparative adjective like bolji. Puno is very common in casual, everyday speech, while mnogo is slightly more standard for writing.