Learn Hardcore Korean: seonsaengnimi haksaengdeurege suyeongeul gareuchyeoyo. - The teacher teaches swimming to students.

이 is the subject marker used after a noun ending in a consonant. It indicates that 선생님 (teacher) is the subject of the sentence. If the noun ends in a vowel, you would use 가 instead.

Why do we use 학생들에게 instead of just 학생들?

에게 (in plural form 들에게) is the dative marker, indicating the indirect object (“to the students”). It shows the recipients of the teaching action. Without this marker, 학생들 would simply be “students” in the nominative or topic sense, not clearly the recipients.

What’s the difference between 학생에게, 학생한테, and 학생께?

학생에게 is neutral and standard for “to the student.”
학생한테 is more colloquial/informal.
학생께 is the honorific form, used when the recipient deserves extra respect (e.g., elders or superiors).

Why is 수영을 used here?

을 is the object marker, indicating that 수영 (swimming) is the direct object of the verb 가르치다 (to teach). It tells us what is being taught.

What form of the verb is 가르쳐요?

가르쳐요 is the present tense polite form (the -요 form) of 가르치다 (to teach). It’s appropriate for everyday polite speech.

Could we omit the object marker 을 in casual speech?

Yes, in very casual or fast speech you might drop 을, saying 수영 가르쳐요, but it’s clearer and more grammatically correct to keep the marker in standard Korean.

What is the basic word order in this sentence?

Korean typically follows Subject–Object–Verb (SOV). Here:
Subject: 선생님이
Indirect Object: 학생들에게
Direct Object: 수영을
Verb: 가르쳐요

Why don’t we use the topic marker 은/는 with 선생님?

Using 이/가 emphasizes who is performing the action (the teacher). If you used 은/는 (선생님은), you’d shift focus to the teacher as a topic—implying contrast or setting up context—rather than simply stating “The teacher teaches swimming to the students.”