jeoneun keopie seoltangeul an neoheoyo.

저 is the polite way to say I.
는 is the topic marker, so 저는 means something like as for me.

In this sentence, 저는 sounds natural because the speaker is talking about a personal habit or preference. If you used 제가, it would put more focus on who does not add sugar, as if answering Who doesn’t add sugar? rather than simply stating your own preference.

Here, 에 marks the place or target that something goes into. So 커피에 설탕을 넣어요 is literally put sugar into coffee.

In English, we often say in coffee, but Korean uses 에 to show the destination or container receiving the object. So 커피에 is very natural here.

설탕을 uses the object marker 을 because sugar is the thing being added.
The verb 넣다 means to put in / to add, and the thing you put in is the direct object.

So:

• 커피에 = into the coffee
• 설탕을 = sugar
• 넣어요 = put/add

안 is the short, common way to say not before a verb or adjective.
So 안 넣어요 means do not put in or don’t add.

There is also a longer negative form:

• 설탕을 안 넣어요
• 설탕을 넣지 않아요

Both mean basically the same thing. The 안 form is very common in everyday speech.

The dictionary form is 넣다. It means to put in, to insert, or to add, depending on context.

In this sentence, the most natural English meaning is add:

• 커피에 설탕을 넣어요 = add sugar to coffee

So 넣어요 is the polite present-form of 넣다.

It can be either, depending on context. Korean present tense often covers both general habits and what someone is doing now.

So this sentence can mean:

• I don’t put sugar in coffee as a general preference, or
• I’m not putting sugar in the coffee in the current situation

Without extra context, many people would understand it as a general habit or preference.

This is in the standard polite style.
Two things make it polite:

• 저 is the polite word for I
• -어요 is a polite verb ending

You could compare:

• 저는 커피에 설탕을 안 넣어요. = polite, everyday
• 저는 커피에 설탕을 안 넣습니다. = more formal
• 나는 커피에 설탕 안 넣어. = casual

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

So all of these can work:

• 저는 커피에 설탕을 안 넣어요.
• 커피에 설탕을 안 넣어요.
• 설탕 안 넣어요.

The full sentence is useful for learners because it clearly shows all the parts, but in real conversation Korean often drops what is obvious.

Sometimes, yes, especially in casual speech.
For example, people often drop the object marker 을/를 in conversation:

• 커피에 설탕을 안 넣어요.
• 커피에 설탕 안 넣어요.

Both are natural. But keeping the particles is clearer and is usually better for learning and careful speech.

The verb usually comes at the end, but the order before the verb is somewhat flexible.
The most neutral order here is:

• 저는 커피에 설탕을 안 넣어요.

You may also hear variations like:

• 저는 설탕을 커피에 안 넣어요.

That is still grammatical, but the emphasis can feel a little different. The original sentence is the most straightforward and natural version for a beginner to learn.

넣다 literally means put in, and that naturally extends to add in English. So for sugar, milk, or another ingredient, 넣다 is very common and easy to understand.

You may also hear other verbs in related situations, but 넣다 is a very standard and useful choice. For a learner, it is a great verb to know because it works in many everyday situations.