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gakkou ni ikutoki, basu wo norikaeru hituyou ga arimasu.

学校がっこう　に　行いくとき、　バス　を　乗のり換かえる　必要ひつよう　が　あります。gakkou ni ikutoki, basu wo norikaeru hituyou ga arimasu.
When going to school, I need to transfer buses.
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Questions & Answers about gakkou ni ikutoki, basu wo norikaeru hituyou ga arimasu.

Why is に used after 学校, and could you use へ instead?

に marks the destination when you say “to school.” It tells us where you’re going. へ can also indicate direction (“towards school”), but:

  • に is more definite: you arrive at school.
  • へ is more general: you head in the direction of school (less emphasis on arrival).

So 学校に行くとき means “when going to [and arriving at] school.” You could say 学校へ行くとき, but the nuance shifts slightly toward “heading toward” school.

Why is the verb 行く in the dictionary form before とき, and why not the past tense 行ったとき?

When you attach とき (“when”) to a verb in dictionary form (plain non-past), it refers to the time “just as” or “on the way” to doing that action. Here:

  • 学校に行くとき = “when (I) am going to school” (the act of going is in progress).

If you used the past tense:

  • 学校に行ったとき = “when (I) went to school” (after arriving at school).

They describe different timings: non-past before the action completes, past after it completes.

What function does とき have in this sentence?

とき means “when.” It turns the preceding clause (学校に行く) into an adverbial clause, specifying the time for the main clause. The structure is:
[time clause] + とき, [main clause]
→ “When I go to school, I need to transfer buses.”

Why is バス followed by を instead of に or で?

The verb 乗り換える (“to transfer [from one vehicle to another]”) takes a direct object: the thing you are transferring. You transfer the bus (i.e., change from one bus to another), so you mark バス with を.

  • バスを乗る means “ride a bus,” but
  • バスを乗り換える means “change buses.”

Particles に and で are not used with 乗り換える in this way.

How is 乗り換える必要がある structured, and what does it literally mean?

It’s built as:

  1. 乗り換える (dictionary verb) modifies
  2. 必要 (noun “need/necessity”)
  3. が (subject particle)
  4. ある (verb “there is”)

Literally: “There is a need to transfer.” In smoother English: “You need to transfer (buses).”

Why is the copula あります used instead of です in 必要がある?
  • 必要 is a noun meaning “necessity.”
  • You state the existence of that necessity with ある (“there is”).
  • Using です would make 必要です, which describes something as “necessary” but doesn’t explicitly express “there is a need.”

Both are possible, but 必要がある (+ あります for politeness) is the standard way to say “have to / need to.”

Why isn’t there a の between 乗り換える and 必要? Would 乗り換えるのが必要だ be correct?

Japanese allows a dictionary-form verb to directly modify a noun: 乗り換える必要. Adding の changes the grammar:

  • 乗り換えるのが必要だ is also correct and means the same thing, but here the structure is: [verb + の] as a noun, then が必要だ.
  • If you wanted a compound noun you could also say 乗り換えの必要がある, using the noun 乗り換え
    • の
      • 必要.

Each version is grammatical; the original (乗り換える必要がある) is just more concise.

Could I express the necessity differently, for example using ～なくてはいけない or ～なきゃ?

Yes. Common colloquial or polite alternatives:

  • 学校に行くとき、バスを乗り換えなくてはいけません。 (polite)
  • 学校に行くとき、バスを乗り換えなきゃいけない。 (casual)
  • 学校に行くとき、バスを乗り換えないといけない。 (neutral)

They all convey the same meaning: “You have to transfer buses when going to school.”