Learn Russian: Я всегда стараюсь внимательно слушать, чтобы понимать учителя. - I always try to listen carefully in order to understand the teacher.

When the person doing the main action and the purpose action is the same ("I try" and "I understand"), we use чтобы followed directly by an infinitive. It translates perfectly to "in order to understand."

Why do we use the imperfective verbs слушать and понимать here instead of their perfective forms?

The clue is the word всегда (always). It shows that this is a regular, repeating habit rather than a one-time completed action. Habitual actions almost always require imperfective verbs.

Why is the word for teacher spelled учителя here instead of учитель?

The teacher is the direct object being understood, which means the word must be in the accusative case. Because учитель is a masculine, animate noun (a living person), its accusative form looks exactly like its genitive form, taking the -я ending.

Is the comma before чтобы strictly necessary?

Yes! In Russian punctuation, you must always separate the two halves of a complex sentence with a comma. This means you will virtually always need a comma right before connecting words like чтобы and что.