Learn Portuguese (Portugal): O professor está a ler um poema.

In European Portuguese, ongoing actions (like "-ing" words in English) are formed using the verb estar (to be) + a + the infinitive (unconjugated) verb. Here, está (is) + a + ler (to read) combines to mean "is reading".

Why use está instead of é for "is" in this sentence?

Both verbs mean "to be", but we use estar for temporary states and actions happening right now. You will always use estar when making this ongoing action structure, never ser (the verb for é).

Why is it um poema? Doesn't the "a" at the end of the word make it feminine?

Usually, yes! But poema is a tricky exception. Many Portuguese words ending in "-ema" (like poema, problema, and sistema) are actually masculine. That is why it takes the masculine article um instead of the feminine uma.

Does professor only refer to a university professor?

No, in Portuguese, professor is the standard word for a male teacher at any educational level, from primary school up to university. If the teacher were a woman, the sentence would start with A professora.