kurai michi o aruku toki, kuruma ni chuuishimasu.

暗い is the plain い-adjective form, and that is the form used directly before a noun.

• 暗い 道 = a dark road
• 赤い 車 = a red car

You only use 暗く in other grammar patterns, such as:

• 道が暗くなる = the road gets dark
• 暗くて見えない = it’s dark, so I can’t see

So in this sentence, 暗い is correct because it is simply modifying 道.

In Japanese, を does not only mark a direct object. It can also mark the place that someone moves through or along.

So:

• 道を歩く = walk along the road
• 公園を散歩する = take a walk through the park
• 橋を渡る = cross the bridge

Here, 道 is the route or space being traversed, so を is natural.

This is a very common question.

With とき, the tense of the verb before it depends on the timing of the action relative to that time.

• 歩くとき = when walking / when you walk / when you are about to walk
• 歩いたとき = when I walked / when I had walked / after walking

In this sentence, the meaning is a general situation or habit:

• When walking on a dark road, I am careful about cars.

So 歩くとき fits better.

A simple way to remember it:

• dictionary form + とき: before or during that action / general case
• past form + とき: after that action happened

とき means time or when.

The clause before it works like a modifier:

• 暗い道を歩くとき = the time when one walks on a dark road
• more naturally in English: when walking on a dark road

So the first part sets the situation, and the second part gives the main action:

• 暗い道を歩くとき、 = when walking on a dark road,
• 車に注意します。 = I pay attention to cars / I watch out for cars.

Because 注意する commonly uses に for the thing you must be careful about.

• 車に注意する = be careful of cars
• 足元に注意する = watch your step
• 健康に注意する = pay attention to your health

So に marks the target of your attention.

English learners often expect を, but with this verb, に is the standard particle in this meaning.

No. In this sentence, 注意します means:

• pay attention
• be careful
• watch out

So 車に注意します means:

• I watch out for cars
• I am careful about cars
• I pay attention to cars

Be careful: 注意 can mean attention or warning, and 注意する can sometimes mean to warn someone in other contexts. But here it clearly means to be careful / to watch out.

します is the polite non-past form of する.

• 注意する = plain form
• 注意します = polite form

Japanese often uses the polite form in neutral example sentences, textbooks, and everyday conversation when speaking politely.

Also, the non-past form in Japanese can express:

• a habit
• a general truth
• a future action

Here it sounds like a habitual or general statement:

• When I walk on a dark road, I watch out for cars.

The subject is omitted because Japanese often leaves out information that is understood from context.

Depending on the situation, this could mean:

• I watch out for cars
• you should watch out for cars
• people watch out for cars

In a standalone textbook sentence, the most natural reading is usually:

• I watch out for cars when I walk on a dark road.

But Japanese does not have to state 私は unless it is needed for contrast or clarity.

Japanese nouns do not normally show singular vs. plural the way English does.

So 車 could mean:

• a car
• cars
• sometimes more generally vehicles, depending on context

In this sentence, English usually sounds most natural as cars:

• When walking on a dark road, I watch out for cars.

That is not because Japanese explicitly marked it as plural. It is just the most natural English translation.

The comma is there to separate the time-setting phrase from the main statement.

• 暗い道を歩くとき、車に注意します。

It works like this:

• When walking on a dark road, | I watch out for cars.

Japanese commas are often used more flexibly than English commas, but here it helps readability by clearly separating the when clause from the main clause.