oneul jeonyeoge mwo meogeulji ajik mos jeonghaesseoyo.

Here, 에 marks the time when something happens. So 오늘 저녁에 means this evening / tonight in the sense of at dinner time or for this evening.

A very natural way to think about it is:

• 오늘 = today
• 저녁 = evening / dinner
• 오늘 저녁에 = this evening / tonight / for dinner tonight

In Korean, time expressions sometimes appear with or without 에, depending on the word and the situation. With 오늘, 내일, 어제, you often do not use 에 by themselves. But when a more specific time noun like 저녁 is part of the phrase, 에 is common and natural.

뭐 is the casual spoken form of 무엇, both meaning what.

So:

• 무엇 = what (more formal, dictionary-like, careful)
• 뭐 = what (very common in speech)

In this sentence, 뭐 먹을지 means what to eat.

You will hear 뭐 much more often in everyday Korean than 무엇.

먹을지 is part of an embedded question, meaning something like what to eat or whether to eat depending on context.

In this sentence:

• 뭐 먹을지 = what to eat

The structure is:

• verb stem + -(으)ㄹ지

This often expresses an uncertain choice or question inside a larger sentence.

So:

• 뭐 먹을지 정하다 = decide what to eat

Why 먹을지 and not 먹는지?

• 먹을지 points to a choice about a future action: what to eat
• 먹는지 usually means whether someone is eating / whether it is eaten, more like checking a fact

So here, because the speaker is deciding on a future meal, 먹을지 is the natural form.

It is an embedded question clause.

Breakdown:

• 뭐 = what
• 먹을지 = whether to eat / what to eat

Together, 뭐 먹을지 functions like a noun clause: what to eat.

That whole clause becomes the object of 정하다:

• 뭐 먹을지 정하다 = to decide what to eat

This is very common in Korean. Similar patterns include:

• 어디 갈지 정했어요 = I decided where to go.
• 누구를 만날지 모르겠어요 = I don’t know who to meet.
• 언제 출발할지 아직 몰라요 = I still don’t know when to leave.

아직 means still or yet.

In this sentence, it gives the idea of not yet:

• 아직 못 정했어요 = haven’t decided yet

It shows that the action is unfinished up to now.

Common uses:

• 아직 안 왔어요 = He/She hasn’t come yet.
• 아직 몰라요 = I still don’t know.
• 아직 못 먹었어요 = I haven’t eaten yet.

So 아직 is an important clue that the decision remains undecided at the present moment.

This is a very common learner question.

Both 안 정했어요 and 못 정했어요 can be translated as I haven’t decided, but the nuance is a little different.

• 안 정했어요 = I did not decide / I haven’t decided

  • more neutral
  • can sound like a simple fact, or even a deliberate non-action depending on context

• 못 정했어요 = I couldn’t decide / I haven’t managed to decide

  • suggests difficulty, inability, or that the decision has not been successfully made yet

In this sentence, 아직 못 정했어요 sounds very natural because deciding what to eat can feel like something the speaker has not managed to settle on yet.

So the nuance is something like:

• I still haven’t been able to decide what to eat for dinner tonight.

In everyday Korean, 못 is often used this way even when English would simply say haven’t decided yet.

정했어요 comes from the verb 정하다, which means to decide, to determine, or to settle on.

Conjugation:

• dictionary form: 정하다
• verb stem: 정하-
• past form: 정했어요

Because 하다 verbs contract this way:

• 정하였어요 → 정했어요

So:

• 정했어요 = decided
• 못 정했어요 = couldn’t decide / haven’t decided

This is one of the most important points for English speakers.

Korean often uses a past form to talk about whether something has been completed or not by now. So 못 정했어요 literally looks like couldn’t decide / did not decide, but in context with 아직, it naturally means:

• haven’t decided yet

The focus is on the result up to the present moment:

• the decision has not been made

This is very normal in Korean.

Compare:

• 아직 숙제 안 했어요 = I haven’t done my homework yet.
• 아직 못 골랐어요 = I haven’t chosen yet.
• 아직 못 정했어요 = I haven’t decided yet.

So even though English often uses have not + past participle, Korean commonly uses a past-style form with words like 아직.

Yes, the subject is omitted, which is very normal in Korean.

The sentence does not explicitly say I, but it is understood from context. So the implied subject is usually:

• 저는 / 제가 = I

Korean often leaves out subjects and objects when they are obvious. In English, you usually need to say them, but in Korean you often do not.

So this sentence naturally means something like:

• I still haven’t decided what to eat for dinner tonight.

But depending on context, it could also mean we haven’t decided.

정했어요 is in the standard polite style, often called the -아요/어요 style.

It is polite and very common in everyday conversation. You can use it with:

• strangers
• coworkers
• people you do not speak casually with
• most normal daily situations

Compare:

• 못 정했어요 = polite
• 못 정했어 = casual
• 못 정했습니다 = more formal

So this sentence is polite but not stiff.

Yes, but it would sound more formal or less conversational.

Compare:

• 오늘 저녁에 뭐 먹을지 아직 못 정했어요. = natural everyday speech
• 오늘 저녁에 무엇을 먹을지 아직 못 정했어요. = more formal, more written, more careful

Both are correct, but 뭐 is much more natural in ordinary conversation.

Also notice that with 무엇, Korean often includes 을/를:

• 무엇을 먹을지

With 뭐, speakers often leave the object marker out:

• 뭐 먹을지

That omission is very common in speech.

In theory, you could say:

• 뭘 먹을지
• 뭐를 먹을지
• 뭐 먹을지

All of these can mean what to eat.

In real conversation, 뭐 먹을지 is very natural, and Korean often drops object markers when the meaning is clear.

A few notes:

• 뭐를 is possible, but less elegant in everyday speech
• 뭘 is a contraction of 뭐를
• 뭐 먹을지 is simple and common

So the sentence sounds natural even without 을/를.

Yes, many speakers would say it that way too.

Both are natural:

• 오늘 저녁에 뭐 먹을지 아직 못 정했어요.
• 오늘 저녁 뭐 먹을지 아직 못 정했어요.

The version with 에 sounds slightly more explicitly marked for time, but in everyday conversation, omitting it is common.

This is one of those places where Korean is flexible.

Both are understandable, but 뭐 먹을지 아직 못 정했어요 is the more standard and natural structure.

Why?

Because 정하다 usually takes an embedded clause with -ㄹ지:

• 뭐 먹을지 정하다 = decide what to eat

By contrast, 뭐 먹을까 is a direct self-question meaning what shall we/I eat?

So:

• 뭐 먹을지 아직 못 정했어요 = I haven’t decided what to eat yet.
• 뭐 먹을까 = What should I/we eat?

You can combine 먹을까 with verbs like 고민하다 or 생각하다 more naturally:

• 오늘 저녁에 뭐 먹을까 고민 중이에요. = I’m thinking about what to eat tonight.

But with 정하다, -ㄹ지 is the usual choice.

Yes. 저녁 can refer to:

• the evening as a time of day
• dinner / the evening meal

In this sentence, both ideas overlap naturally. The speaker is talking about the evening meal, so English might express it as:

• for dinner tonight
• this evening
• tonight

The exact wording in English depends on the translation shown to the learner, but in Korean this is very natural and slightly flexible.

A helpful breakdown is:

• 오늘 저녁에 = this evening / for dinner tonight
• 뭐 먹을지 = what to eat
• 아직 = yet / still
• 못 정했어요 = haven’t decided / couldn’t decide

So the sentence structure is:

[time] + [embedded question clause] + [still/yet] + [have not managed to decide]

This is a very common Korean pattern:

• 언제 갈지 아직 못 정했어요. = I haven’t decided when to go yet.
• 누구를 부를지 아직 못 정했어요. = I haven’t decided who to invite yet.
• 어디에서 만날지 아직 못 정했어요. = I haven’t decided where to meet yet.