Learn Hardcore Japanese: watasi ha yoru no densya no oto ni odorokimasita. - I was surprised by the sound of the train at night.

は is the topic marker, indicating that 私 (“I”) is what the sentence is about. If you used が, you’d be emphasizing the subject as new information or focusing on 私 as opposed to someone else. Here, the speaker simply sets “I” as the topic before describing what happened.

What is the function of the first の in 夜の電車 and the second の in 電車の音?

The first の connects 夜 (“night”) to 電車 (“train”), meaning “train of the night” or “night train.” The second の links 電車 to 音 (“sound”), giving “the sound belonging to (or produced by) the night train.” Stacked の chains like this are common in Japanese for descriptive purposes.

Why is に used after 音 instead of を?

The verb 驚く is intransitive and takes the cause or stimulus marked by に. You’re not acting upon 音; rather, you are surprised by (caused by) the sound. If you used を, it would imply a transitive action on the sound, which doesn’t fit here.

What is the dictionary form of 驚きました, and how do I make the plain past form?

The dictionary (base) form is 驚く. The polite past 驚きました comes from the masu-stem 驚き plus ました. To get the plain past, change く to いた, giving 驚いた.

What is the nuance difference between 驚く and びっくりする?

Both mean “to be surprised,” but 驚く feels more formal or literary, while びっくりする is colloquial and mimics the onomatopoeic sound of a sudden shock. You’ll hear びっくりする more in everyday conversation.

Can 私は be omitted here? When would you drop it?

Yes. In Japanese, the subject or topic is often omitted if it’s clear from context. If you’re already talking about yourself, you could just say 夜の電車の音に驚きました. Including 私は adds clarity or emphasis on who experienced the surprise.

Is there a more casual way to express this idea?

Sure. In casual speech you might say: 夜の電車の音にびっくりした。 That uses びっくりする in its plain past form and drops the polite ending.

How do you read the entire sentence in kana?

わたしはよるのでんしゃのおとにおどろきました。