Learn Portuguese (Portugal): Desculpe, posso usar a casa de banho?

This is the main rule for the verb poder (can/may). When you say you can do something, you conjugate poder to match the person doing the action (here, posso means I can), and you leave the second verb in its original dictionary form (usar, meaning to use).

Where is the word for I in this sentence?

In Portuguese, the verb ending usually tells you who the subject is. The verb posso specifically means I can. You could say eu posso, but native speakers almost always drop the eu (I) because the verb form already makes it perfectly clear.

Why does the sentence use Desculpe instead of Desculpa?

Desculpe is the formal way to say excuse me or sorry. Because you are likely asking a stranger, waiter, or shop worker to use their facilities, you should use this polite formal version rather than the informal Desculpa.

What does casa de banho literally mean?

It literally means house of bath. In Portugal, casa de banho is the standard, everyday way to ask for the bathroom, restroom, or toilet.