Learn Italian: Avevo un diario pieno di segreti. - I had a diary full of secrets.

Avevo is the imperfetto (imperfect) tense of avere. We use the imperfetto here because having a diary was a continuous, ongoing state in the past, rather than a single completed event. It describes how things used to be or sets a scene.

Why does the word pieno end in -o?

Pieno (full) is an adjective describing un diario (a diary). Since diario is a masculine singular noun, the adjective must match it by taking the masculine singular ending -o.

How does the preposition work in the phrase pieno di segreti?

In Italian, you use the preposition di (of) after pieno to say what something is filled with. If you were saying 'full of the secrets', you would combine di with the plural article to make dei, but because you mean general 'secrets' here, the plain di is perfect.

Could I use uno instead of un for diario?

No, you must use un. Even though diario might look a bit different, it starts with a simple consonant ('d'). You only use uno for masculine words starting with 'z', 'ps', 'gn', or 's' followed by another consonant (like uno specchio).