achimbabeul meogeumyeon ojeone deol pigonhae.

Yes, 아침밥 means breakfast.

Literally:

• 아침 = morning
• 밥 = cooked rice, but very often also meal

So 아침밥 is literally something like morning meal, and in normal English that is just breakfast.

A few nuance points:

• 아침밥 sounds natural and everyday.
• A more formal word is 아침 식사.
• In casual speech, Koreans often use 밥 to mean a meal, not just rice.

을 is the object marker.

In this sentence, 아침밥 is the thing being eaten, so it takes the object marker:

• 아침밥을 먹다 = to eat breakfast

Korean often marks the direct object this way:

• 밥을 먹다 = eat a meal / eat rice
• 사과를 먹다 = eat an apple
• 물을 마시다 = drink water

Here, 아침밥 ends in a consonant, so the object marker is 을. If the noun ended in a vowel, it would usually be 를.

먹으면 comes from 먹다 + -으면.

Breakdown:

• 먹다 = to eat
• 먹- = verb stem
• -으면 = if / when

So 먹으면 means if one eats or when one eats.

In this sentence, it expresses a general result:

• 아침밥을 먹으면 오전에 덜 피곤해 = If you eat breakfast, you are less tired in the morning.

A useful grammar note:

• Use -으면 after a consonant-ending stem
• Use -면 after a vowel-ending stem

Examples:

• 먹다 → 먹으면
• 가다 → 가면

It can mean either, depending on context.

With -면 / -으면, Korean often does not sharply separate if and when the way English sometimes does.

In this sentence, the meaning is something like a general tendency:

• If / when I eat breakfast, I’m less tired in the morning

So the feeling is not necessarily a one-time future event. It can also mean a habitual result:

• Whenever I eat breakfast, I’m less tired in the morning

Because Korean often leaves out the subject when it is already understood from context.

So this sentence could mean:

• If I eat breakfast, I’m less tired in the morning
• If you eat breakfast, you’re less tired in the morning
• If people eat breakfast, they’re less tired in the morning

Korean does this very naturally. If the speaker wanted to be explicit, they could say:

• 나는 아침밥을 먹으면 오전에 덜 피곤해. = If I eat breakfast, I’m less tired in the morning.

But very often, the subject is simply omitted.

오전 means the morning, specifically A.M. / before noon.

The 에 here is a time marker, so:

• 오전에 = in the morning / during the morning / in the A.M.

So:

• 오전에 덜 피곤해 = am less tired in the morning

The particle 에 is commonly used with times:

• 아침에 = in the morning
• 세 시에 = at three o’clock
• 월요일에 = on Monday

Both are possible, but they are a little different in nuance.

• 아침에 = in the morning, in a more everyday sense
• 오전에 = in the morning, more specifically before noon, and a bit more clock-based or slightly more formal

In this sentence, 오전에 can emphasize the time period after breakfast, during the morning hours.

So:

• 아침밥을 먹으면 아침에 덜 피곤해 = natural
• 아침밥을 먹으면 오전에 덜 피곤해 = also natural, with a slightly more specific before noon feel

덜 means less.

Here it modifies 피곤해:

• 덜 피곤해 = less tired

This is important because it does not mean not tired at all.

Compare:

• 덜 피곤해 = less tired
• 안 피곤해 = not tired

So the sentence means breakfast reduces the tiredness, not necessarily that it removes it completely.

This is a speech-level issue.

• 피곤하다 = dictionary form
• 피곤해요 = polite everyday speech
• 피곤해 = casual / intimate speech

So the sentence as written is casual:

• 아침밥을 먹으면 오전에 덜 피곤해.

A polite version would be:

• 아침밥을 먹으면 오전에 덜 피곤해요.

The meaning is the same. Only the level of politeness changes.

For English-speaking learners, it is often easiest to think of 피곤하다 as meaning to be tired or be tired.

In school-style Korean grammar, words like 피곤하다 are often treated as descriptive verbs rather than adjectives in the English sense. That is why they conjugate like verbs:

• 피곤하다
• 피곤해
• 피곤해요
• 피곤하면

So even though it translates like an adjective in English, it behaves like a Korean predicate that conjugates.

Yes. A few natural variations are:

• 아침밥을 먹으면 오전에 덜 피곤해요.
  Same meaning, but polite.

• 아침 식사를 하면 오전에 덜 피곤해요.
  More formal because 아침 식사 is more formal than 아침밥.

• 아침을 먹으면 오전에 덜 피곤해요.
  Also natural. In everyday Korean, 아침을 먹다 can mean to eat breakfast.

• 아침밥을 먹으면 아침에 덜 피곤해요.
  Uses 아침에 instead of 오전에.

All of these are natural; the differences are mostly about formality and nuance.