oneureun haesbichi ganghaeseo seonkeurimeul kkok ballaya haeyo.

은/는 is the topic particle, so 오늘은 means something like as for today or today, ....

In this sentence, 오늘은 sets the scene:

• 오늘은 = today
• then the speaker says what is true today: the sunlight is strong, so you need sunscreen.

If you said 오늘이, it would sound like today is the grammatical subject in a different kind of sentence, which is not what is happening here.

So:

• 오늘은 = topic / setting
• 햇빛이 = subject of the clause that follows

Here, 햇빛이 is the subject of 강하다 (to be strong/intense).

So the structure is:

• 햇빛이 강하다 = the sunlight is strong

The particle 이/가 marks the thing that is doing or being something in the clause. In this sentence, what is strong? 햇빛.

So:

• 오늘은 = topic
• 햇빛이 = subject
• 강해서 = because it is strong

강해서 comes from 강하다 (to be strong/intense) plus -아서/어서, which often means because or so.

Breakdown:

• 강하다 = to be strong
• 강해서 = because it is strong / being strong, so...

In this sentence:

• 햇빛이 강해서 = because the sunlight is strong

This grammar connects the reason to the result:

• reason: 햇빛이 강해서
• result: 선크림을 꼭 발라야 해요

So -아서/-어서 is a very common way to express cause:

• 비가 와서 집에 있어요. = Because it’s raining, I’m staying home.
• 배가 아파서 약을 먹었어요. = Because my stomach hurt, I took medicine.

Because 선크림 is the object of the verb 바르다 (to apply/spread on).

• 바르다 is an action verb
• you apply something
• that something gets 을/를

So:

• 선크림을 바르다 = to apply sunscreen

Compare:

• 선크림이 좋아요. = The sunscreen is good.
  Here, 선크림 is the subject.
• 선크림을 발라요. = I apply sunscreen.
  Here, 선크림 is the object.

꼭 means surely, definitely, or without fail. In this sentence, it adds the idea of really making sure you do it.

So:

• 선크림을 발라야 해요 = you have to apply sunscreen
• 선크림을 꼭 발라야 해요 = you really must / you should definitely apply sunscreen

It gives extra emphasis. It does not create the obligation by itself; the obligation comes from -아/어야 해요.
꼭 just strengthens it.

Other examples:

• 약을 꼭 먹어야 해요. = You really must take the medicine.
• 내일 꼭 와요. = Please definitely come tomorrow.

This comes from the grammar -아/어야 하다, which means must, have to, or need to.

Breakdown:

• dictionary form: 바르다 = to apply
• stem used here: 발라-
• 발라야 하다 = have to apply
• 발라야 해요 = polite everyday form

So the full structure is:

• 바르다 → 발라야 하다 → 발라야 해요

This pattern is extremely common:

• 가야 해요 = have to go
• 먹어야 해요 = have to eat
• 공부해야 해요 = have to study

So in your sentence, 발라야 해요 is the part that expresses necessity.

This is because 바르다 is a 르 irregular verb.

With many 르 verbs, when you add a vowel-starting ending like -아/어, two things happen:

1. 르 changes
2. an extra ㄹ gets added to the previous syllable

So:

• 바르다
• → 발라요
• → 발라야 해요

That is why you get 발라- rather than 바르어-.

A few similar examples:

• 모르다 → 몰라요 = don’t know
• 고르다 → 골라요 = choose
• 부르다 → 불러요 = call / sing

So 발라야 is just the natural conjugated form of 바르다.

Grammatically, -아/어야 해요 is closer to must / have to / need to than to a mild should.

So:

• 선크림을 발라야 해요 = You have to apply sunscreen

However, in real conversation, Korean often sounds softer than a direct English must, especially with 해요 politeness. Depending on context, it can feel like:

• You need to apply sunscreen
• You should really put on sunscreen

So the grammar expresses obligation/necessity, but the exact English wording can vary depending on tone and context.

해요 is the polite everyday style in Korean. It is very common in normal conversation when speaking respectfully but not formally.

So:

• 발라야 해요 = polite, natural, everyday
• 발라야 합니다 = more formal
• 발라야 해 = casual/plain speech to someone close

This means the sentence sounds polite and natural in daily use, such as when talking to:

• a friend politely
• a coworker
• someone you don’t know very well
• a customer in casual service speech

Korean word order is more flexible than English because particles show each word’s role. But some orders sound more natural than others.

The original sentence is very natural:

• 오늘은 햇빛이 강해서 선크림을 꼭 발라야 해요.

You could sometimes move parts for emphasis, for example:

• 햇빛이 강해서 오늘은 선크림을 꼭 발라야 해요.

That still makes sense, but the nuance changes slightly because the emphasis changes.

What usually stays important is:

• the reason clause before the result clause
• the verb at the end

So while Korean allows some movement, the original version is a very standard and natural way to say it.

Yes. In Korean, 강하다 is very natural for things like sunlight, wind, smell, force, intensity, and effect.

So:

• 햇빛이 강하다 = the sunlight is strong / intense

This is a normal Korean expression. English learners might expect words like bright, but Korean often uses strong/intense for sunlight when talking about how powerful it feels or how much effect it has.

Compare:

• 햇빛이 밝다 would focus more on brightness
• 햇빛이 강하다 focuses more on intensity/strength

In this sentence, 강하다 fits very well because strong sunlight is the reason you need sunscreen.