Learn Hardcore Korean: neoneun hyudaeponeul jaju sseo?

는 is the topic marker. It marks 너 (“you”) as the topic of the sentence, roughly “As for you …”. It sets up what you’re going to ask or say about “you.”

Why is 휴대폰을 used with 을?

을 is the object particle. Because 휴대폰 ends in a consonant, you attach 을 to indicate that “mobile phone” is the direct object of the verb: “use the phone.”

What does 써 mean in this context?

써 is the informal present tense form of the verb 쓰다, meaning “to use.” So 너는 휴대폰을 자주 써? asks, “Do you use your phone often?”

Why is 쓰다 conjugated to 써 instead of 쓰어?

When a verb stem ends in ㅡ, Korean drops the ㅡ before adding 어. Thus 쓰 + 어 contracts to 써.

Why isn’t there a 요 at the end of the sentence?

This is informal casual speech (해체), used among close friends or younger people. To make it polite, you’d add 요, e.g., 너는 휴대폰을 자주 써요?

What’s the difference between 써 and 사용해?

써 comes from 쓰다 and is the everyday, casual way to say “use.”
사용해 comes from 사용하다 and feels more formal or written; you’ll hear it in polite/formal contexts.

Can 너는 be omitted here?

Yes. Korean often drops topic/subject pronouns when they’re understood from context. You can simply say 휴대폰을 자주 써?

What does 자주 mean, and where can it go in the sentence?

자주 means “often.” It’s an adverb that usually sits right before the verb: 휴대폰을 자주 써? Placing it elsewhere (e.g., before the object) is possible for emphasis but less common.

What level of formality is this sentence, and how would you make it polite?

The given sentence is informal casual (해체), used among close friends or younger people.
To make it polite, you could say:
• 너는 휴대폰을 자주 써요? (friendly polite, 해요체)
• 휴대폰을 자주 사용하세요? (more formal polite, 하십시오체)