Trovo l’avviso sulla porta.

Italian is a pro-drop language, which means subject pronouns (io, tu, lui/lei, etc.) can be left out when the verb ending clearly indicates the subject. Here, trovo ends in -o, so it’s unambiguously first person singular. Adding io would be correct but redundant:
• (Io) trovo l’avviso sulla porta.

Trovo is the present indicative, first-person singular form of the regular -are verb trovare (“to find”). The conjugation pattern for trovare in the present is:
• io trovo
• tu trovi
• lui/lei trova
• noi troviamo
• voi trovate
• loro trovano

1. In Italian, nouns generally require an article (definite or indefinite). You can’t say avviso on its own as easily as English “notice.”
2. The definite article l’ here signals that you’re referring to a specific notice already known or visible. If it were any notice, you’d use un avviso:
   • Ho visto un avviso sulla porta.

Sulla porta means “on the door.” It’s a fused form of the preposition su (“on”) and the feminine singular definite article la. So:
• su + la porta → sulla porta

All Italian simple prepositions merge with the matching definite article. Here are the combinations with su (“on”):
• su + il → sul
• su + lo → sullo
• su + la → sulla
• su + l’ → sull’ (before vowels)
• su + i → sui
• su + gli → sugli
• su + le → sulle

Both can mean “on,” but there’s a subtle nuance:
• su is more general and handles abstract or physical “on.”
• sopra emphasizes being directly above or over something.
In many cases they’re interchangeable:
– Il libro è sul tavolo.
– Il libro è sopra il tavolo.
However, su is more idiomatic for “on” in set phrases, and sopra may imply a looser “over” sense.