電車が止まるか止まらないかのうちに乗客が立ち上がった。
- The passengers stood up the moment the train stopped.

電車が止まるか止まらないかのうちに乗客が立ち上がった。 meaning: The passengers stood up *before* the train stopped or didn't stop.

The sentence follows a fairly complex structure typical of N1 grammar, containing several embedded clauses. The core structure is: Subject + Time Phrase + Verb. ⚫︎ The subject is 乗客 (passengers). ⚫︎ The verb is 立ち上がった (stood up). ⚫︎ The time phrase is 電車が止まるか止まらないかのうち (while the train is stopping or not stopping). This time phrase uses the key grammar points explained below.

This pattern is used to express uncertainty or alternatives. ⚫︎ 止まるか止まらないか (whether it stops or doesn't stop) uses the question particle か to link two contradictory possibilities. The second verb 止まらない (doesn't stop) is in the negative form. ⚫︎ The structure is: Verb (plain form) + か + Verb (negative plain form) + か. ⚫︎ The entire phrase 止まるか止まらないか is then nominalized, acting as a noun phrase that functions as part of the larger time phrase.

のis a particle that nominalizes verbs or adjectives, turning them into nouns. ⚫︎ In this sentence, it appears as 止まるか止まらないかの, nominalizing the phrase about stopping. This creates a noun phrase that can be modified by the particle うち. ⚫︎ The structure is: Verb/Adjective (plain form) + の. ⚫︎ This nominalization allows the phrase to function as the subject, object, or modifier within a larger sentence structure.

This is a fixed expression indicating a period of time or a condition. It expresses that something happens within the context of something else. ⚫︎ ～のうちに means "while," "during," or "within" the specified time or condition. It emphasizes that the main action occurs *within* the scope of the event expressed in the phrase preceding のうちに. ⚫︎ In this sentence, the passengers stood up のうちに, which means the main action of standing up happened while the train was stopping or not stopping. ⚫︎ The structure is: [Noun Phrase/Verb (plain form) + の] + うちに.

Understanding verb conjugations is crucial. ⚫︎ The verb 止まる (to stop) is in its plain form. This is because the entire phrase 止まるか止まらないか is functioning as a noun phrase in this case. ⚫︎ The verb 立ち上がった (stood up) is in the past tense, indicating that the action has already been completed. ⚫︎ The pattern uses the plain form, which is used in a variety of situations.

While が (however/but) is not the core meaning of the sentence, it offers a slight degree of nuance. ⚫︎ The が particle does not appear at the end of the sentence. ⚫︎ It is included within a larger grammatical structure. ⚫︎ The use of が is for contrast to create a smooth transition between different parts of a sentence.