Learn Spanish (Latin America): El profesor es serio.

We use es (from the verb ser) because we are talking about a permanent personality trait: the teacher is a serious person in general. If we used está (from the verb estar), it would mean he is just acting serious right now, perhaps because the class was misbehaving.

Why is the word serio instead of seria?

In Spanish, describing words (adjectives) must match the gender of the person or thing they describe. Since el profesor refers to a male teacher, the adjective takes the masculine ending -o. For a female teacher, the sentence would be La profesora es seria.

The word profesor doesn't end in -o. How do we know it is masculine?

While many masculine words end in -o, words that end in -or are also almost always masculine. The word el before it is another big clue! To make professions ending in -or feminine, you usually add an -a to the end, making it la profesora.