Learn Italian: Posso usare la tua penna?

In Italian, when you use a modal verb like potere (can or to be able to), the action verb that follows it must be in the infinitive form, which is the unconjugated dictionary form ending in -are, -ere, or -ire. Because posso is already conjugated to mean 'I can', the second verb usare (to use) stays as an infinitive.

Where is the word for 'I' in this sentence?

The word for 'I' is io, but in Italian, subject pronouns are usually dropped. The verb form posso already means 'I can', so saying io posso is unnecessary unless you want to emphasize that it is specifically you asking.

Why does it say la tua penna instead of just tua penna?

In Italian, possessive adjectives like tua (your) almost always need a definite article like la (the) right in front of them. You are literally saying 'the your pen'. You must include the article for objects, even though English does not do this.

How is this structured as a question without changing the word order?

In Italian, you do not need to swap the subject and verb to ask a question. You simply take the normal statement (Posso usare la tua penna meaning 'I can use your pen') and add a question mark at the end. When speaking, you just raise the pitch of your voice at the end of the sentence.