doma wieseo tomatoreul jareul ttae kareul josimhaeseo sseoyo.

도마 means cutting board, and 위 means top/surface. So 도마 위 is on the cutting board or more literally on top of the cutting board.

The particle 에서 is used because this is the place where the action happens.
Here, the action is cutting.

• 도마 위에 = on the cutting board
• 도마 위에서 = on the cutting board, as the location where you do something

So 에서 is the natural choice because someone is performing an action there.

Because there are really two verbs in the sentence:

1. 토마토를 자를 때 = when cutting the tomatoes
2. 칼을 조심해서 써요 = use the knife carefully

Each verb has its own object:

• 토마토를 goes with 자르다 = to cut
• 칼을 goes with 쓰다 = to use

So even though it is one full sentence in English, Korean is showing the structure of both parts very clearly.

자르다 means to cut.

To say when doing X, Korean often uses verb stem + -(으)ㄹ 때.

So:

• 자르다 → verb stem 자르-
• stem ends in a vowel sound, so add -ㄹ 때
• 자르- + ㄹ 때 = 자를 때

This means when cutting or when you cut.

A useful comparison:

• 먹다 → 먹을 때 = when eating
• 가다 → 갈 때 = when going
• 자르다 → 자를 때 = when cutting

Not necessarily. In this kind of sentence, -(으)ㄹ 때 usually means when doing or when you do in a general sense.

So 토마토를 자를 때 is best understood as:

• when cutting tomatoes
• when you cut tomatoes

It is often used for:

• general habits
• instructions
• repeated situations

So it does not have to mean strict future time.

조심해서 comes from 조심하다, which means to be careful or to take care/caution.

The form is:

• 조심하다
• 조심해서

Here, -해서 connects it to the next verb and gives the sense of:

• being careful, ...
• carefully ...
• with caution ...

So 칼을 조심해서 써요 means something like:

• use the knife carefully
• be careful when using the knife

It is functioning adverbially, modifying 써요.

Both 쓰다 and 사용하다 can mean to use, but 쓰다 is much more natural in everyday Korean for many common things, including tools.

So:

• 칼을 써요 = use a knife
• 펜을 써요 = use a pen
• 돈을 써요 = spend/use money

사용하다 sounds more formal or technical. It is not wrong, but in a simple everyday sentence like this, 쓰다 is the more natural choice.

This is because of how ㅡ verbs conjugate.

Start with:

• 쓰다 → stem 쓰-

When adding -어요, the ㅡ usually drops:

• 쓰 + 어요 → 써요

So 써요 is the normal polite present form of 쓰다.

This same pattern happens with other verbs too:

• 크다 → 커요
• 끄다 → 꺼요
• 쓰다 → 써요

-요 makes the sentence polite.

So 써요 is a polite everyday form. In this sentence, it gives the feeling of:

• polite advice
• an instruction
• a general safety reminder

Even though it looks like present tense, Korean often uses this kind of present polite form for general instructions.

So this is not only I use the knife carefully. In context, it can naturally mean:

• Use the knife carefully
• You should use the knife carefully

Korean very often leaves out the subject when it is obvious from context.

In this sentence, the hidden subject is probably:

• you
• one
• people in general

That is very normal in Korean. If the sentence is part of instructions or advice, Korean usually does not need to say you explicitly.

Good question. Both can appear in sentences about knives, but they do different things.

In 칼을 조심해서 써요, the main verb is 쓰다 = to use, so the knife is the object of that verb:

• 칼을 쓰다 = to use a knife

If you used 칼로, that would mark the knife as the tool/means:

• 칼로 자르다 = to cut with a knife

So compare:

• 토마토를 칼로 잘라요 = cut the tomatoes with a knife
• 칼을 조심해서 써요 = use the knife carefully

Both are correct, but they fit different sentence structures.

The given order is very natural, but Korean word order is more flexible than English.

This sentence is organized as:

• location: 도마 위에서
• time/action clause: 토마토를 자를 때
• main object + adverbial + verb: 칼을 조심해서 써요

This is a very normal Korean pattern: background information first, main action later.

You can sometimes move parts around for emphasis, but not completely freely. For example, 토마토를 도마 위에서 자를 때 is also understandable, but the original version is smooth and natural.

They are close, but not exactly the same in feel.

• 조심히 is an adverb meaning carefully
• 조심해서 literally feels more like being careful and...

So both can work in similar situations:

• 칼을 조심히 써요
• 칼을 조심해서 써요

But 조심해서 is very common in spoken instructions and safety reminders. It sounds natural and conversational.

In isolation, it is most naturally understood as an instruction or piece of advice.

That is because:

• it uses polite -요
• it talks about a general situation
• it has a safety-related expression: 조심해서

So although the grammar looks like a plain present-tense sentence, in real use it often means something like:

• Be careful using the knife when cutting tomatoes on the cutting board
• Use the knife carefully when cutting tomatoes on the cutting board

This is very common in Korean: a present polite sentence can function like a mild command or instruction.