naneun hyeonggwangpeneul du gae sassneunde hananeun jibe dugo hananeun gabange neoheo.

나는 marks I as the topic: As for me...

That is very natural when the speaker is just telling you what they do with the highlighters.
If you used 내가, it would sound more like you are emphasizing I specifically, often in contrast to someone else.

So here:

• 나는 = neutral topic-setting
• 내가 = more subject-focused or contrastive

In Korean, counted nouns usually follow the pattern:

noun + number + counter

So 형광펜 두 개 literally looks like highlighter two units.

That is the normal Korean order. English puts the number first, but Korean usually puts the noun first when using counters.

Also, both of these are natural:

• 형광펜을 두 개 샀어
• 형광펜 두 개를 샀어

Both mean I bought two highlighters. Korean allows the particle to attach either before or after the counter phrase in many cases.

개 is the general counter for things, so it is very common and easy to use.

For writing instruments, a more specific counter like 자루 can also be used:

• 형광펜 두 자루

But in everyday speech, 개 is very common and sounds completely natural.

So 두 개 is a normal learner-friendly and conversational choice.

Not always. -는데 can mean several things depending on context.

Here, it is not a strong but. It is more like:

• I bought two highlighters, and...
• I bought two highlighters, so...
• I bought two highlighters, and here’s what I do with them...

It sets up background for what follows.

So in this sentence, 샀는데 connects the first idea to the explanation about the two highlighters.

Here 는 is doing a contrastive job.

The pattern 하나는 ... 하나는 ... means:

• one ...
• the other one ...

The speaker is contrasting the two highlighters:

• one stays at home
• one goes in the bag

This is a very common use of 은/는 in Korean: not just topic marking, but also contrast.

You could also say:

• 하나는 집에 두고 다른 하나는 가방에 넣어

That makes the contrast even more explicit: one ..., the other ...

Because once 형광펜 has already been mentioned, Korean usually does not repeat it if the meaning is obvious.

So 하나는 means one of them.

A fuller version would be something like:

• 형광펜 하나는 집에 두고 다른 하나는 가방에 넣어

But repeating the noun is often unnecessary. Korean frequently leaves out repeated words when the listener can easily understand them.

Yes. 두고 comes from 두다, which means to put, to leave, or to keep depending on context.

Here:

• 두다 = to leave / keep
• 두고 = leaving/keeping ... and then...

The -고 ending connects this action to the next one.

So:

• 하나는 집에 두고
• 하나는 가방에 넣어

means roughly:

I leave one at home and put one in my bag.

Because 에 marks the place where something is placed or ends up.

With verbs like 두다 and 넣다, 에 is very common because it shows the destination or location of the object:

• 집에 두다 = leave it at home
• 가방에 넣다 = put it in a bag

This is different from 에서, which usually marks the place where an action happens.

Here the focus is not doing the action at home or in the bag, but where the highlighter ends up.

Yes. 넣어 is the casual, non-polite form.

It is used in:

• casual conversation with close friends
• self-talk
• diary style
• speech where politeness is not needed

The polite version would be:

• 넣어요

The plain dictionary form would be:

• 넣다

So this sentence is in casual speech style.

By itself, 넣어 could be a command in the right context.

But in this sentence, because the subject 나는 is stated at the beginning, it is understood as the speaker describing their own action or habit, not telling someone else what to do.

So here it reads as a statement, not an imperative.

Context matters a lot in Korean endings like this.

Because each clause can show its own time reference.

• 샀는데 = the buying happened in the past
• 두고 / 넣어 = what the speaker does with them now, regularly, or as their usual arrangement

So the sentence can mean something like:

I bought two highlighters, and I keep one at home and put one in my bag.

If you wanted everything to sound like one completed past sequence, Korean would more likely use a past ending at the end too, such as:

• 하나는 집에 두고 하나는 가방에 넣었어

So the tense mix is not a mistake; it reflects the meaning.