Learn Hardcore Korean: chinguwa hamkke bada-eseo suyeonghaeyo.

와 is the vowel-ending form of the coordinate particle 와/과, which can mean “and” or “with.” Here it indicates “with my friend,” showing that the action is performed together with a friend.

Why is 함께 included in the sentence; doesn’t 친구와 already mean “together”?

함께 literally means “together” and adds emphasis on the joint action. While 친구와 identifies who you’re with, 함께 highlights that you’re actually doing the activity as a group. You can omit it—친구와 바다에서 수영해요 still works—but 함께 makes the sense of togetherness more explicit.

What role does 에서 play in 바다에서?

에서 is the locative particle that marks where an action takes place. By saying 바다에서, you specify that the swimming happens “at the sea.”

Why is the verb ending -해요 used here? Are there other polite forms?

-해요 is the present-tense polite ending (해요체) for verbs like 수영하다. Depending on formality and tense, you could also say:
• 수영합니다 (formal polite)
• 수영해 (casual/informal)
• 수영했어요 (past polite)

Why isn’t a subject like “I” or “we” included in the sentence?

Korean often drops subjects when they’re clear from context. Here, it’s understood that the speaker (and the friend) are doing the swimming. If you want to be explicit, you could add 저희가, 우리가 or 나는 at the front, but it’s natural to omit it.

Can I replace 바다 with 해변? What’s the difference?

바다 refers to the sea or ocean (the water itself), whereas 해변 means the beach or shoreline (the land beside the water). You swim in 바다, so 바다에서 수영해요 is correct. If you say 해변에서 놀아요, it implies playing on the beach rather than swimming.

Is word order flexible in Korean? Could I say 바다에서 친구와 함께 수영해요?

Yes. Because each phrase has its own particle, you can switch 친구와 함께 and 바다에서 without changing the meaning. Just remember the verb stays at the end:
• 친구와 함께 바다에서 수영해요
• 바다에서 친구와 함께 수영해요