houchou o tsukau toki wa, manaita ga ugokanai you ni ki o tsukemasu.

A natural reading is:

ほうちょう を つかう とき は、まないた が うごかない よう に き を つけます。

Without the added spaces, it would normally be written like this:

包丁を使うときは、まな板が動かないように気をつけます。

A very natural English gloss is When using a kitchen knife, be careful to make sure the cutting board does not move.

Because 包丁 is the direct object of 使う.

• 包丁を使う = to use a kitchen knife
• を marks the thing being used

So this part is very straightforward:

• 包丁 = kitchen knife
• を使う = use it

Because 使うとき means when using or when you use a knife, referring to that general time/situation.

With とき, the tense before it helps show the timing:

• 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, the meaning is a general safety instruction, so 使うとき is the natural choice.

The は marks the whole time phrase as the topic or frame for the sentence.

So:

• 包丁を使うとき = when using a kitchen knife
• 包丁を使うときは = as for when using a kitchen knife / when using a kitchen knife,

It sets up the situation, and then the rest of the sentence tells you what to do in that situation.

Because まな板 is the subject of 動かない here.

• まな板が動かない = the cutting board does not move

Using が is natural because it marks the thing whose state is being described.

If you said まな板は動かない, that would sound more like a contrast or separate topic, such as As for the cutting board, it doesn't move. That is possible in some contexts, but here が is the most natural way to say so that the cutting board does not move.

It means so that it does not move or in such a way that it will not move.

This pattern is very common:

• plain verb + ように
• negative plain verb + ように

In this sentence:

• 動く = move
• 動かない = does not move
• 動かないように = so that it does not move

So the speaker is paying attention with the goal of preventing the cutting board from moving.

Because the meaning is preventive.

• 動くように = so that it moves
• 動かないように = so that it does not move

Since a moving cutting board is dangerous, the sentence uses the negative form to show the desired result: keep it from moving.

ように is very common when talking about a desired result or state, especially with things like:

• possibility
• ability
• natural outcomes
• intransitive verbs
• prevention

Here, 動く is an intransitive verb, and the idea is make sure the cutting board does not move. That fits ように very well.

If you tried to use ために, it would sound less natural in this sentence. ために is often used more for deliberate purpose with controllable actions by the same subject.

So 動かないように気をつけます is the natural Japanese pattern.

It means to be careful, to pay attention, or to take care.

Literally:

• 気 = spirit / mind / attention
• 気をつける = attach your attention to something

In this sentence, a very natural translation is:

• be careful that the cutting board does not move
• make sure the cutting board does not move

So it is not just a vague be careful. It specifically means paying attention to prevent that problem.

Japanese often omits the subject when it is understood from context.

So 気をつけます could mean:

• I am careful
• you should be careful
• one should be careful

In a safety instruction like this, English often uses you or an impersonal style:

• When using a knife, be careful that the cutting board does not move.
• When using a knife, make sure the cutting board does not move.

The Japanese does not need to state the subject explicitly.

It breaks down like this:

• 包丁を使うときは = when using a kitchen knife
• まな板が動かないように = so that the cutting board does not move
• 気をつけます = be careful / pay attention

So the full structure is:

[time/situation] + [desired result] + [main action/attitude]

Japanese often puts background information first and the main verb at the end, so this word order is very normal.

Yes. Both are used.

• 気をつけます
• 気を付けます

The hiragana version is very common, and many textbooks prefer it because it is simple and easy to read. The meaning is the same.