Learn Italian: Il mio cane aveva molto coraggio.

In Italian, you use the imperfetto (like aveva) to describe an ongoing state, a habit, or a physical/mental characteristic in the past. Here, you are describing the dog's ongoing personality, rather than a single, completed event.

Why is there an Il before mio cane? Doesn't mio already mean my?

Yes, but in Italian, possessives like mio almost always need a definite article (il, la, etc.) right before them. The main exception is for singular family members (like mio padre for my father). Since a dog is an animal and not a grammatical family member, it follows the standard rule: il mio cane.

Why isn't there a word for of after molto?

In English, we say a lot of, but in Italian, molto works directly as an adjective meaning much or many. So molto coraggio literally translates to much courage, which means you don't need a preposition like di (of) at all.

Could I say the dog was molto coraggioso instead?

Absolutely! Just like in English, you can say he had a lot of courage (aveva molto coraggio) or you can use the verb essere to say he was very courageous (era molto coraggioso). Both sound perfectly natural in Italian.