otouto ha zitensya ni noru toki, rouka de ha naku genkan de herumetto wo kaburimasu.

弟は marks younger brother as the topic of the sentence.

So the sentence is talking about as for my/your/his younger brother... and then says what he does.

A learner might expect が, but は is natural here because:

• the brother is the main topic
• the sentence is making a statement about his habit or usual behavior

Using が would sound more like you are identifying the brother specifically as the one who does this, while は feels more like speaking about him in general.

The verb 乗る uses the particle に for the thing you get on or ride.

So:

• 自転車に乗る = ride a bicycle
• バスに乗る = get on / ride a bus
• 電車に乗る = ride a train

English uses a direct object with ride, so ride a bicycle can make learners expect を, but Japanese treats it differently. With 乗る, the vehicle is marked by に.

Before nouns like とき (time/when), Japanese normally uses the plain form.

So:

• 自転車に乗るとき = when riding a bicycle / when he rides a bicycle

Even though the main sentence ends politely with かぶります, the verb before とき stays in plain form.

This is very common:

• 行くとき = when going
• 食べるとき = when eating
• 寝るとき = when sleeping

It means when he rides a bicycle or when riding a bicycle.

The phrase modifies the rest of the sentence by giving the situation or time frame:

• 自転車に乗るとき、...かぶります
• When he rides a bicycle, ... he puts on a helmet

It does not necessarily mean the exact instant of getting on the bicycle. In natural English, it usually means when he is going to ride or for bicycle riding.

ではなく means not ... but rather ... or not in/at ...

Here:

• 廊下ではなく = not in the hallway
• 玄関で = but in the entranceway / at the entrance

The で is the location particle, and は adds contrast:

• 廊下で = in the hallway
• 廊下では = in the hallway, as opposed to somewhere else

So ではなく is a very natural way to create a contrast:

• Xではなく、Yで...
• not in X, but in Y...

The は is a contrastive は.

Normally, で marks the place where an action happens:

• 玄関でかぶります = he puts it on in the entranceway

When you change で to では, it adds emphasis or contrast:

• 廊下ではなく = not in the hallway

So the sentence is contrasting two locations:

• not in the hallway
• but in the entranceway

This kind of は often appears when comparing or correcting alternatives.

Because で marks the place where an action happens.

The action here is ヘルメットをかぶります (puts on / wears a helmet), and that action happens in the entranceway.

So:

• 玄関でヘルメットをかぶります = he puts on a helmet in the entranceway

Compare:

• 学校で勉強する = study at school
• 部屋で食べる = eat in the room

Because ヘルメット is the thing being put on, and かぶる takes a direct object marked by を.

So:

• 帽子をかぶる = wear/put on a hat
• ヘルメットをかぶる = wear/put on a helmet

This is different from English in one important way: かぶる is specifically used for things worn on the head.

It can suggest either, depending on context.

かぶる can mean:

• to put on something on the head
• to wear something on the head

In this sentence, because there is a location contrast:

• not in the hallway
• but in the entranceway

it strongly sounds like he puts on the helmet in the entranceway.

So in context, puts on is probably the most natural interpretation, even though wears is also possible in some situations.

Because once the topic 弟は is established, Japanese usually does not repeat it unless needed.

So after:

• 弟は...

the rest of the sentence is still understood to be about the younger brother:

• 自転車に乗るとき、廊下ではなく玄関でヘルメットをかぶります。

Japanese often leaves out words that are already clear from context. This is very normal and makes sentences sound natural.

It breaks down like this:

• 弟は = as for my/your/his younger brother
• 自転車に乗るとき = when he rides a bicycle
• 廊下ではなく = not in the hallway
• 玄関で = in the entranceway
• ヘルメットをかぶります = puts on/wears a helmet

So the structure is basically:

[Topic] + [when-clause] + [not X but Y location] + [main action]

This kind of word order is very common in Japanese: background information comes first, and the main verb comes at the end.

Yes, 廊下でなく is possible, but 廊下ではなく sounds more explicitly contrastive.

Compare:

• 廊下でなく、玄関で... = not in the hallway, but in the entranceway
• 廊下ではなく、玄関で... = not in the hallway, but rather in the entranceway

The version with は makes the contrast clearer and is very natural when correcting or contrasting locations.

By itself, it most naturally sounds like a habitual or general action.

That is because:

• 弟は sets up a general topic
• 自転車に乗るとき often describes what someone usually does in that situation
• かぶります in non-past polite form can describe regular behavior

So the sentence is likely saying what he normally does whenever he rides a bicycle. If you wanted to make it clearly about one specific occasion, the context would usually make that clear.