daidokoro de houchou o tsukau toki wa, ki o tsukete kudasai.

で marks the place where an action happens.

So in 台所で, the meaning is in the kitchen or at the kitchen, as the location where the action takes place.

• 台所で 包丁を使う = use a knife in the kitchen
• If you used に instead, it would sound wrong here, because に is not normally used to mark the place where an action like 使う happens.

A quick comparison:

• 台所にいる = be in the kitchen
• 台所で料理する = cook in the kitchen

So here, since using is an action, で is the correct particle.

を marks the direct object of the verb.

The verb here is 使う = to use, and the thing being used is 包丁 = kitchen knife.

So:

• 包丁を使う = use a knife

This is very standard Japanese verb-object structure:

• 本を読む = read a book
• 水を飲む = drink water
• 包丁を使う = use a knife

とき means when or at the time of.

So:

• 包丁を使うとき = when using a knife / when you use a knife

In Japanese, the clause before とき describes the time:

• 食べるとき = when eating / when you eat
• 寝るとき = when going to sleep / when you go to bed
• 包丁を使うとき = when using a knife

So the structure is:

[short clause] + とき

which means when [that clause happens].

This is an important grammar point.

With とき, the form before it helps show the timing relationship.

• 使うとき = when using / when you are about to use / when you use
• 使ったとき = when you used / when you have used

In this sentence, 使うとき is natural because the warning applies to the general situation of using a knife.

So the sentence is talking about the time while using or when using a knife, not after the action is completed.

A simple contrast:

• 日本へ行くとき、パスポートを持っていく。
  When going to Japan, take your passport.
• 日本へ行ったとき、寿司を食べた。
  When I went to Japan, I ate sushi.

The は makes 包丁を使うとき the topic or frame for the rest of the sentence.

So:

• 包丁を使うときは、気をつけてください。

means something like:

• As for when you use a knife, please be careful.
• More naturally: When using a knife, please be careful.

The は adds a slight sense of in that situation or at least in that case.

Without は, 包丁を使うとき、気をつけてください is also possible and still natural.
With は, the warning feels a little more clearly framed or emphasized.

This expression is made from:

• 気をつける = to be careful / to pay attention
• 気をつけて = the て-form
• ください = please do ...

So:

• 気をつけてください = please be careful

This is a very common polite request pattern:

• 見てください = please look
• 待ってください = please wait
• 気をつけてください = please be careful

Also, 気をつける is an idiomatic expression. Literally, it looks like attach/apply your attention/mind, but the natural meaning is simply be careful or take care.

Yes, but it is a polite command/request, not a harsh one.

Japanese often uses てください to tell someone to do something politely:

• 座ってください = please sit
• 読んでください = please read
• 気をつけてください = please be careful

So this sentence sounds like a normal polite instruction, warning, or reminder. It would be appropriate in places like:

• safety instructions
• a teacher speaking to students
• a parent speaking politely
• a sign or announcement

It is not rude.

Japanese often leaves the subject unstated when it is obvious from context.

In this sentence, the implied subject is usually you:

• When using a knife in the kitchen, please be careful.

That you is not written, because Japanese often omits subjects that can be understood naturally.

This is very common. For example:

• 気をつけてください。 = Please be careful.
• 早く来てください。 = Please come early.

The listener is understood automatically.

Not exactly.

Because of the particle placement, the most direct structure is:

• 台所で = in the kitchen
• 包丁を使う = use a knife

So the sentence means:

• When using a knife in the kitchen, please be careful.

If you wanted kitchen knife as one combined idea, Japanese would more naturally treat that as a noun phrase, not as 台所で 包丁を使う.

Here, 台所 is the place of the action, not a modifier of 包丁.

They are the same word.

• とき = written in hiragana
• 時 = written in kanji

So this sentence could also be written as:

台所で包丁を使う時は、気をつけてください。

Both are correct.

In learning materials, hiragana is often used:

• to make reading easier
• when the writer wants a softer or simpler look
• when the kanji may be considered a little harder for the reader

So there is no grammar difference here.

The spaces are just for learners. Normal Japanese writing usually does not use spaces between words.

Learner version:

台所　で　包丁　を　使う　とき　は、　気　を　つけて　ください。

Normal writing:

台所で包丁を使うときは、気をつけてください。

The spaces help beginners see the word and particle boundaries more clearly.

Yes. That would still be natural, but the tone changes.

• 気をつけてください = polite, more formal
• 気をつけて = more casual or direct

So:

• 台所で包丁を使うときは、気をつけてください。
  polite
• 台所で包丁を使うときは、気をつけて。
  casual, like to a friend, child, or family member

Both mean Be careful when using a knife in the kitchen, but ください makes it more polite.

Yes, 台所 means kitchen, but there is a nuance.

• 台所 is a traditional/native Japanese word
• キッチン is also common, especially in modern everyday speech and housing-related contexts

In many cases, both can refer to a kitchen, but 台所 can feel a little more traditional or slightly more old-fashioned depending on context.

Still, it is a completely normal word, and in a sentence like this it sounds natural.