Learn Italian: Cucino il pesce nel burro. - I cook the fish in butter.

In Italian, you often use the definite article for substances and ingredients when talking about them in a general sense. So, butter becomes il burro. When you combine the preposition in (in) with il (the), they merge into the combined word nel.

I have also learned the verb cuocere for 'to cook'. Why use cucino here?

Both cucinare and cuocere mean to cook, but they have different focuses. Cucinare is the general act of preparing food or making a meal, which fits perfectly here. Cuocere focuses on the physical transformation of the food through heat (like baking, boiling, or roasting).

Why isn't there a subject pronoun like io (I) at the beginning of the sentence?

In Italian, you can usually drop the subject pronoun. The -o ending on the verb cucino already makes it perfectly clear that 'I' am the one doing the cooking. You would only add io if you wanted to emphasize that you are doing it, rather than someone else.

How do I know if pesce means one fish or multiple fish?