saendeuwichireul meogeumyeonseo mul han keopdo gati masyeosseo.

먹으면서 comes from the verb 먹다 (to eat) plus -면서, which means while doing, as one does, or sometimes and at the same time.

So:

• 먹다 → 먹으면서
• 먹으면서 = while eating

In this sentence, it connects the two actions:

• eating the sandwich
• drinking a cup of water

So the idea is: I drank a cup of water too while eating the sandwich.

Usually -면서 shows that two actions overlap in time, often by the same subject.

In this sentence:

• 샌드위치를 먹으면서
• 물 한 컵도 같이 마셨어

the speaker is doing both actions in the same general time period: eating and drinking.

It does not always have to mean every second is perfectly simultaneous. It often means something like:

• while
• as
• at the same time

A very important point: -면서 normally implies the same person does both actions unless the sentence clearly says otherwise.

Good question. 같이 means together, along with, or also in combination.

So even though 먹으면서 already gives the idea of doing something while eating, 같이 adds the feeling of:

• along with it
• together with the sandwich
• at the same time too

Here it makes the sentence sound more natural and emphasizes that the water was drunk with the sandwich.

Without 같이, the sentence would still be grammatical:

• 샌드위치를 먹으면서 물 한 컵도 마셨어.

But 같이 adds a smoother, more natural with it / along with that nuance.

물 한 컵 means one cup of water.

Korean often uses counters when talking about amounts or units. For drinks, common expressions are:

• 물 한 컵 = one cup of water
• 물 한 잔 = one glass/cup of water
• 물 한 병 = one bottle of water

Here:

• 한 = one
• 컵 = cup
• 물 한 컵 = a cup of water

You would not normally say 물 하나 for one water in this context, because liquids are usually counted by containers or units, not by plain number words alone.

도 usually means also, too, or sometimes even.

In this sentence, 물 한 컵도 most naturally means:

• a cup of water too
• also a cup of water

So the speaker is not only eating the sandwich, but also drinking water.

The nuance depends on context:

• In a neutral context, 도 = also/too
• In some contexts, it can feel like even

Here, the most natural reading is I also drank a cup of water.

Because 도 replaces other particles in many cases.

Normally you might expect:

• 물 한 컵을 마셨어 = I drank a cup of water

But when you add 도, it often takes the place of 을/를:

• 물 한 컵도 마셨어 = I drank a cup of water too

So this is completely normal Korean.

Compare:

• 사과를 먹었어 = I ate an apple
• 사과도 먹었어 = I ate an apple too

You usually do not say 사과를도.

Because they are marking the nouns in different ways.

• 샌드위치를: the object marker 를 shows sandwich is the thing being eaten
• 물 한 컵도: 도 is used instead of the object marker and adds the meaning too/also

So:

• 샌드위치를 먹으면서 = while eating the sandwich
• 물 한 컵도 마셨어 = I drank a cup of water too

This difference is very common in Korean.

In Korean, when one verb is connected to another, the tense is often shown only on the final verb.

So:

• 먹으면서 = while eating
• 마셨어 = drank

Because the final verb is in the past tense, the whole sentence is understood as past:

• While eating the sandwich, I drank a cup of water too.

You do not need to make both verbs past tense.

This is very normal in Korean subordinate clause structure.

마셨어 is in the informal casual style, often called 해체.

Compare:

• 마셨어 = casual/informal
• 마셨어요 = polite
• 마셨다 = plain/written/dictionary-style statement

So this sentence sounds like something you would say casually to:

• a friend
• a younger person
• someone you are close to

A polite version would be:

• 샌드위치를 먹으면서 물 한 컵도 같이 마셨어요.

Yes, Korean word order is somewhat flexible, especially as long as the particles and meanings are clear.

The original sentence is:

• 샌드위치를 먹으면서 물 한 컵도 같이 마셨어.

You might also hear:

• 샌드위치를 먹으면서 같이 물 한 컵도 마셨어.
• 물 한 컵도 샌드위치를 먹으면서 같이 마셨어.

But the original order sounds very natural and easy to follow.

In general:

• the -면서 clause often comes first
• the main action comes later

That is the most common pattern for learners to use safely.

Not in this sentence.

같이 can mean either:

1. together with someone
2. together / along with / at the same time

Here, it means the second one. There is no person mentioned, so it does not mean with someone else. It means something more like:

• along with that
• together with the sandwich
• at the same time

So the sentence is about combining food and drink, not about another person.

Yes, but the meaning changes a little.

• 먹으면서 = while eating
• 먹고 = and/after eating

Compare:

• 샌드위치를 먹으면서 물 한 컵도 같이 마셨어.
  = I drank a cup of water too while eating the sandwich.

• 샌드위치를 먹고 물 한 컵도 같이 마셨어.
  = I ate the sandwich and also drank a cup of water.
  This does not emphasize simultaneity as much.

So if you want the idea of during the eating, -면서 is better.

It is a loanword from English sandwich.

In Korean it is written:

• 샌드위치

And in this sentence:

• 샌드위치를
• 샌드위치 + 를

Because the noun ends in a vowel sound, it takes 를 rather than 을.

Many food words in Korean are loanwords, especially for Western foods, so this is very common.

Yes, it sounds natural.

It has a very everyday spoken feel:

• 샌드위치를 먹으면서 = while eating a sandwich
• 물 한 컵도 같이 마셨어 = I also drank a cup of water with it

A native speaker could easily say this in conversation.

If you wanted a slightly more neutral or polished version, you could also say:

• 샌드위치를 먹으면서 물도 한 컵 같이 마셨어.
• 샌드위치를 먹으면서 물 한 컵도 마셨어.

But the original sentence is already perfectly normal.