Breakdown of Я опаздываю, поэтому должен бежать.
я
I
поэтому
so (therefore)
опаздывать
to be late
должен
must, have to (должен, должна, должны)
Questions & Answers about Я опаздываю, поэтому должен бежать.
Why do we use бежать instead of бегать here?
Бежать is the unidirectional verb for running. Because the speaker is late, they need to run to a specific destination right now, in one direction. You would use бегать if you were talking about running for exercise or running around aimlessly.
Why is опаздываю a verb when the English translation says 'I am late' (which uses an adjective)?
In Russian, опаздывать is an action verb meaning 'to be arriving late' or 'to be running late'. So я опаздываю literally means 'I am running late' right now as an ongoing process, which is why it uses the present tense.
Could I use the perfective form опоздать in this situation?
Not if you are still on your way! The perfective опоздать means you have already arrived late and the damage is done. Because the speaker is still trying to get there and can still run, the ongoing imperfective опаздываю is the only correct choice.
How does the word должен work in this sentence?
Должен (must / have to) is actually a short-form adjective, not a verb. That means it changes to match the gender and number of the subject. The form должен is masculine, so we know a man is speaking. If a woman were speaking, she would say я должна бежать.