あ、バス、(a. 行かないと　b. 行っちゃった)。How come answer a is not right?


I am currently preparing for the N3 with the 日本語総まとめ books and one question
confuses me:

> あ、バス、(a. 行かないと b. 行っちゃった)。

Supposedly, a. is wrong while b. is correct. But doesn't a. sound more
reasonable, in the sense of "Ah, the bus, I have to go."?

Also, concerning, 行っちゃった, is it common to use 行く when describing the motion of
a vehicle? Wouldn't you rather use 出かける or 出発する?

I am grateful for explanations regarding any of those questions!




This answer extends on a lot of what @Earthliŋ♦ and @T.Allred have already
posted.

## Short answer

The presence of one noun and one verb, in the absence of explicit particles,
may imply that the noun is the subject and the verb belongs to that subject.
Or, it may imply that the noun is the object of the verb. Given that, it comes
down to which answer makes sense logically.

## Long answer

> a. あ、バス、行かないと。

This may imply

「あ、バス（が）、行かないと。」

Logically, however, this doesn't make a lot of sense because it means, "Oh,
the bus, it has to go".

Or, it may imply

「あ、バス（を）、行かないと。」

This doesn't make sense, either. It might mean "Oh, we have to walk down the
bus".

As @T.Allred explained, 「あ、バスだ。行かないと。」would make this answer correct.

For the second choice,

> b. あ、バス、行っちゃった。

Here, one noun, one verb, so again it's implied that the noun can be the
subject or object of the verb. Of the two cases, it makes logical sense for
the noun to be the subject, so this option is correct. This means, "Aw man,
the bus left".

## Response to additional question

> Also, concerning, 行っちゃった, is it common to use 行く when describing the motion
> of a vehicle? Wouldn't you rather use 出かける or 出発する?

Yes, it is common to use 行く when expressing that a car, train, etc., has gone,
especially when you don't want it to have already gone （行ってしまいました/行っちゃった）.

The subject of 出かける is normally a person/people, and, per Earthliŋ♦, it means
"to go out". So, although バスが出かける may be grammatically correct, from a logical
standpoint it might be a little funny.

出発する literally means "to depart". Usage of 出発する, however, is common from a
vehicle operator (driver/pilot) to passengers in the vehicle. It's also common
when talking about vehicles with respect to its schedule. In the case of the
question, 「あ、バス、出発しちゃった」isn't incorrect. However, あ、○○ ~ちゃった implies that the
speaker is talking/thinking to him/herself, or the speaker is talking to a
close friend in a colloquial manner. In such a colloquial situation, as
@Earthliŋ♦ indicated, 出発する doesn't seem natural, much like, "Aw man, the bus
departed" doesn't seem natural, either.




Yes, the 行かないと is saying "I have to go" but evidently that is not what the
question wanted to express. The 行っちゃった is saying that that it went (with a
feeling of regret or disappointment).

> Ah, the bus, it left

行く is a general purpose "go" but 出かける or 出発する would be a more specific
"depart".

It would be hard to answer that question without more context but if the
sentence were to be "Ah, the bus, I gotta go." I feel it would be more natural
to say:

> あ、バス **だ** 。行かないと

