daidokoro de houtyou wo tukau toki ha, yubi wo kegasinai you ni ki wo tukemasu.

で marks the place where an action happens.

• 台所で = in the kitchen
• The action is 包丁を使う (use a knife), and that action happens in the kitchen.

If you used に, it would sound more like a destination or location of existence, not the place where the action is carried out.

Because 包丁 is the thing being used, so it takes を.

• 包丁を使う = use a knife

The particle で can mark a tool, but only when that tool is used to do some other action.

For example:

• 包丁で切る = cut with a knife

So:

• 包丁を使う = the knife is the object being used
• 包丁で切る = the knife is the tool used to cut

とき means when or at the time of.

The phrase:

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

Japanese often puts the modifying clause before the noun it describes. Here, 使う modifies とき.

So literally it is something like:

• the time when one uses a knife

This is a very common question.

• 使うとき = when using / when you use
• 使ったとき = when you used / when you have used / after using

The tense before とき depends on the timing relationship.

Here, the sentence is talking about being careful during the time you are using the knife, so 使うとき is natural.

If you said 使ったとき, it would sound more like when you used it or after using it, which does not fit as well with the warning.

In 包丁を使うときは, the は marks when using a knife as the topic or context.

So the structure is:

• As for when you use a knife, ...
• When using a knife, ...

It sets up the situation being talked about.

Without は, the sentence could still be understandable in some contexts, but は makes the sentence sound more like a general rule or habit.

Yes, けがをする is a very common expression meaning to get injured.

But けがする can also be used with the injured body part as the object:

• 指をけがする = injure your finger / hurt your finger

So here:

• 指をけがしない = do not injure your fingers

This is natural Japanese when talking about specific body parts.

Japanese often leaves out obvious possessors, especially with body parts.

So:

• 指をけがしない naturally means don’t injure your fingers
• It does not need 自分の unless you want extra emphasis or contrast

English usually needs your, but Japanese often does not if the meaning is clear from context.

Here, ように means something like:

• so that... not
• in order not to...
• so as not to...

So:

• 指をけがしないように = so as not to injure your fingers

This pattern is very common:

• verb negative + ように

Examples:

• 忘れないように = so as not to forget
• 遅れないように = so as not to be late

In this sentence, it expresses a precaution or purpose.

気をつけます is the polite form of 気をつける, which means to be careful or to pay attention.

Literally:

• 気 = mind / attention / care
• をつける = attach / apply

So the expression literally feels like apply your attention, but in natural English it means be careful.

In this sentence:

• 指をけがしないように気をつけます
• I am careful so that I do not injure my fingers
• more naturally: I’m careful not to injure my fingers

The sentence ends with 気をつけます, which is polite non-past form.

This is standard polite Japanese. It can express:

• a habit
• a general rule
• something the speaker usually does
• something the speaker intends to do

Here it sounds like a general habit or rule:

• When using a knife in the kitchen, I am careful not to injure my fingers.

It can be written either way:

• けが
• 怪我

In everyday Japanese, けが in hiragana is very common, especially in learner materials and general writing. Using hiragana makes the sentence easier to read.

So both are correct, but けが is very natural.

Yes, to some extent. Japanese word order is flexible as long as the particles make the roles clear.

For example, these are all understandable:

• 台所で包丁を使うときは、指をけがしないように気をつけます。
• 包丁を使うときは、台所で指をけがしないように気をつけます。

However, the original sounds natural and well-organized:

1. place: 台所で
2. situation/topic: 包丁を使うときは
3. main statement: 指をけがしないように気をつけます

So the original order is a very good standard phrasing.

Grammatically, the subject is omitted, so it depends on context.

In many cases, this kind of sentence can mean:

• I am careful not to injure my fingers...
• You should be careful not to injure your fingers...
• People are careful not to injure their fingers...

Because it ends with 気をつけます, it most directly sounds like the speaker is saying what they do: I’m careful...

But Japanese often leaves subjects unstated, especially when they are obvious from context.

A helpful breakdown is:

• 台所で = in the kitchen
• 包丁を使うときは = when using a knife / as for when using a knife
• 指をけがしないように = so as not to injure one’s fingers
• 気をつけます = am careful / pay attention

So the pattern is:

[place] + [when/in what situation] + [what bad thing to avoid] + [be careful]

That is a very common and useful Japanese sentence pattern.