dongsaengeun chareul masil ttae seoltangeul manhi neoheoyo.

은 marks 동생 as the topic of the sentence.

So 동생은 means something like as for my younger sibling or my younger sibling, .... It sets up who the sentence is about.

If you used 동생이, the sentence would sound more like you are specifically identifying the younger sibling as the subject, often with a slightly more neutral or contrast-free feel.

In this sentence, 은 is very natural because the speaker is making a general statement about the younger sibling’s habit:

• 동생은 차를 마실 때 설탕을 많이 넣어요.
• As for my younger sibling, when drinking tea, they put in a lot of sugar.

동생 means younger sibling. By itself, it does not tell you whether the person is male or female.

If you want to be specific, Korean can say:

• 남동생 = younger brother
• 여동생 = younger sister

So in this sentence, 동생 could mean either one, depending on context.

Because there are two different verbs, and each verb has its own object.

The sentence has:

1. 차를 마실 때 → to drink tea
2. 설탕을 많이 넣어요 → put in a lot of sugar

So:

• 차를 is the object of 마시다 (to drink)
• 설탕을 is the object of 넣다 (to put in / add)

This is very normal in Korean when one clause is attached to another.

Because Korean has two different words pronounced 차, and context tells you which one it is.

Here, the phrase is:

• 차를 마실 때 = when drinking tea

Since you drink it and then add sugar, 차 clearly means tea here, not car.

마실 때 means when (someone) drinks or when drinking.

It is made from:

• 마시다 = to drink
• -ㄹ/을 때 = when, at the time of

Because 마시다 ends in a vowel, -ㄹ 때 is attached:

• 마시다 → 마실 때

So:

• 차를 마실 때 = when drinking tea / when one drinks tea

In this sentence, it describes the situation in which the main action happens:

• when the younger sibling drinks tea,
• they add a lot of sugar.

With 때, Korean very often uses the verb stem + -ㄹ/을 때 form to mean when doing or whenever doing, especially in everyday speech.

So:

• 마실 때 = natural and common

마시는 때 is grammatically possible in some contexts, but it sounds much less natural here. In ordinary conversation, 마실 때 is the form learners should expect and use.

A good practical rule is:

• for action verbs with 때, -ㄹ/을 때 is very common for when/whenever doing
• for past meaning, use -았/었을 때
  • 마셨을 때 = when (someone) drank / when I drank

많이 is an adverb meaning a lot, much, or in large quantity.

Here it modifies 넣어요:

• 설탕을 많이 넣어요 = puts in a lot of sugar

So it tells you how much sugar is added.

Compare:

• 설탕을 넣어요 = puts in sugar
• 설탕을 많이 넣어요 = puts in a lot of sugar

넣어요 comes from 넣다, which means to put in, to add, or to place inside.

In this sentence, because the object is sugar, the most natural English meaning is:

• add(s) sugar
• literally, put(s) in sugar

So:

• 설탕을 많이 넣어요 = adds a lot of sugar

-어요 is the polite informal style, often called the 해요 style. It is very common in everyday Korean.

So 넣어요 is a polite way to say:

• puts in
• adds

The plain dictionary form is:

• 넣다

Other style comparisons:

• 넣어요 = polite everyday speech
• 넣어 = casual speech
• 넣습니다 = more formal

Usually it sounds like a general habit.

• 차를 마실 때 설탕을 많이 넣어요 is most naturally understood as whenever my younger sibling drinks tea, they add a lot of sugar

It describes what they normally do.

If you wanted to talk about one specific past occasion, Korean would usually make that clearer, for example with past forms or more context.

Yes. In natural English, you could think of it as:

• when drinking tea
• while drinking tea
• whenever they drink tea

In this sentence, the exact English wording depends on style, but the Korean idea is that the adding of sugar happens in the context of drinking tea.

Korean often leaves out words like my, your, his, her, or their when the meaning is obvious from context.

So Korean says:

• 차를 마실 때 설탕을 많이 넣어요

It does not need to explicitly say:

• their tea
• the sugar
• into the tea

Those relationships are understood naturally from the situation.

This is very common in Korean and can sound much more natural than forcing every detail to be stated.

Yes, exactly.

Korean often omits things that are obvious from context. Here, the listener naturally understands that the sugar is being added to the tea.

So even though Korean literally says:

• puts in a lot of sugar

the natural understood meaning is:

• puts a lot of sugar in the tea
• adds a lot of sugar to their tea

The structure is:

동생은 + 차를 마실 때 + 설탕을 많이 넣어요

Breaking it down:

• 동생은 = as for my younger sibling
• 차를 마실 때 = when drinking tea
• 설탕을 많이 넣어요 = adds a lot of sugar

So the pattern is:

Topic + time/condition clause + main action

This is a very common Korean sentence pattern.