Learn Spanish (Latin America): El profesor está explicando la teoría.

The current lesson is about using llevar + gerund. Since this sentence uses estar + gerund (está explicando), how would we change it to say "The professor has been explaining the theory for an hour"?

You replace the helping verb estar with llevar and add the time period. It becomes: El profesor lleva una hora explicando la teoría. You use estar + gerund to state what is happening right now, but llevar + time + gerund to express how long an ongoing action has been happening.

Could I just say El profesor explica la teoría using the simple present tense?

Yes, but it changes the nuance slightly. Explica can mean he is explaining it now, but it often means he generally explains it (e.g., "The professor explains the theory every semester"). By using está explicando (the present progressive), it makes it crystal clear that the action is in progress right at this exact moment.

Does profesor specifically mean a university professor?

Not necessarily. While "professor" in English usually means someone teaching at a college or university, profesor (and profesora) in Spanish is a general word for "teacher" and is widely used for middle school and high school teachers as well.

Why does teoría have an accent mark on the 'i'?