Learn Italian: Mi hai guardato senza sorridere. - You looked at me without smiling.

In Italian, prepositions like senza (without) are always followed by the infinitive form of a verb, never the gerund. So, to say "without doing something", you must use senza + infinitive.

How do we know who is supposed to be doing the smiling here?

Because this is an implicit clause using an infinitive, the subject of sorridere has to be the exact same as the main verb. Since the main verb is hai guardato (you looked), we automatically know that "you" are the one who wasn't smiling.

The English translation says "looked at me". Why isn't there an Italian word for "at" in this sentence?

In English, you look at someone. In Italian, the verb guardare (to look / to watch) is a direct action. It doesn't need a preposition to connect to its object. The idea of "at" is essentially built into the verb itself, which is why it just takes the direct object pronoun mi.

I am a woman. If I say this sentence, should I change guardato to guardata?