Loro sono vs. Io sono?
Of course when the article is present I know who the sentence is about, but how do I know whether a sentence is "I" or "They" when the article is not there? For instance if I see just "sono" in a sentence how do I know?
It's quite rare to find "sono" alone by itself, usually there are nouns, pronouns or adjectives that tell you whether it's singular or plural (for example, sono italiano vs. sono italiani). But if you do find "sono" by itself, then context will always make it clear. And if even context doesn't make it clear, then you can just assume that it's singular :)
Let's say you're at a party and a friend comes up to you and says: "sono qui". He knows that "sono" is both singular and plural, so if he's telling you this without specifying that he's talking about other people, you can just assume that it's singular. And if he was talking about other people he wouldn't want you to misunderstand and let you think that he was talking about himself, so he'd say "loro sono qui".
You will know that thanks the words that follow 'sono'.
Sono italiano --- 'Italiano' is singular, then the pronoun is 'Io' and the sentence is 'I am Italian'.
Sono studenti --- 'Studenti' is plural then the pronoun is 'Loro' and the sentence is 'They are students' .
Sono stato triste : I was sad.
Sono stato tristi : They were sad.