Learn Croatian: Ovaj laptop je bolji nego moj stari. - This laptop is better than my old one.

The base word is dobar (good). Just like in English where "good" becomes "better", dobar has an irregular comparative form: bolji. Because laptop is a masculine noun, we use the masculine form bolji.

Why is "than" translated as nego here, and could we use something else?

When comparing things in Croatian, you can use nego followed by the same grammatical case as the first item (here, the nominative case moj stari). You could also use od followed by the genitive case, which would look like: Ovaj laptop je bolji od mojeg starog. Both are correct and very common!

Where is the word for "one" in moj stari (my old one)?

Croatian doesn't use a filler word for "one" like English does. You just leave the adjective on its own. Moj stari literally means "my old", but it naturally translates to "my old one" because the word laptop is already understood from the context.

Why are ovaj, bolji, moj, and stari all taking masculine endings?

They all describe or refer to laptop, which is a masculine noun. In Croatian, demonstratives (ovaj), adjectives (bolji, stari), and possessives (moj) must always match the noun they refer to in gender, number, and case. Here, they are all in the masculine singular nominative form.