なったら vs. なると – usage in context


I came across the following exercise – I have to fill in the gap with the
correct word that I have to choose from a series of 4.

> 信号{しんごう}があおに（ ）、道{みち}を渡{わた}っていいです。

I eliminated 2 words and now I'm stuck with: なったら and なると. In my mind both #1
and #2 make sense.

> 1.信号があおになったら、道を渡っていいです。

_If the traffic light becomes / will become blue, it is ok to cross the road._

> 2.信号があおになると、道を渡っていいです。

_When the traffic light will become blue, it is ok to cross the road._

Which one is correct and why?

P.S. The book says なったら is the correct one.




You can only use なったら, not なると.

First, take a look at [this
topic](https://japanese.stackexchange.com/questions/393/differences-
among-%E3%81%9F%E3%82%89-%E3%81%AA%E3%82%89-%E3%82%93%E3%81%A0%E3%81%A3%E3%81%9F%E3%82%89-%E3%81%88%E3%81%B0-etc),
and you see how they exactly describe the difference of と and たら.

> と, ば: The main clause must be a **constant non-volitional reaction** to the
> conditional clause unless the conditional clause shows state or if the
> subjects of the two clauses differ.
>
> ～たら 1. Use when expressing a **one-off (as opposed to constant or general)
> dependency**.
>
> (emphasis added)

Now, why can't you use と in your case? Because the ～て(も)いい is, basically, an
expression giving a permission _ad hoc, each specific time you say the word_.
Thus your example #2 would sound like:

> ?? _You may cross the road now whenever the light turns green._

If you'd like to paraphrase it correctly with と, you'd say:

> 信号があおになると、道を(渡れます/渡ることができます)。 _" You can cross the road when ..."_  
>  信号があおになると、道を渡っていいことになります。 _" When ..., it means you may cross the road
> then."_

It nevertheless **doesn't** mean you can't make a general description with
～ていい. It's free to express a general idea taking the form of commenting about
a particular instance, but as long as you do so, you have to pretend it
through. It's like we could say `Does a wombat bite?` instead of `Do wombats
bite?`, but never `*Do a wombat bite?` (nor `*Does wombats bite?`).

_**Acknowledgement**_ : Thanks to @DariusJahandarie for his help and advice
improving this answer.




You can only use なったら, not なると.

First, take a look at [this
topic](https://japanese.stackexchange.com/questions/393/differences-
among-%E3%81%9F%E3%82%89-%E3%81%AA%E3%82%89-%E3%82%93%E3%81%A0%E3%81%A3%E3%81%9F%E3%82%89-%E3%81%88%E3%81%B0-etc),
and you see how they exactly describe the difference of と and たら.

> と, ば: The main clause must be a **constant non-volitional reaction** to the
> conditional clause unless the conditional clause shows state or if the
> subjects of the two clauses differ.
>
> ～たら 1. Use when expressing a **one-off (as opposed to constant or general)
> dependency**.
>
> (emphasis added)

Now, why can't you use と in your case? Because the ～て(も)いい is, basically, an
expression giving a permission _ad hoc, each specific time you say the word_.
Thus your example #2 would sound like:

> ?? _You may cross the road now whenever the light turns green._

If you'd like to paraphrase it correctly with と, you'd say:

> 信号があおになると、道を(渡れます/渡ることができます)。 _" You can cross the road when ..."_  
>  信号があおになると、道を渡っていいことになります。 _" When ..., it means you may cross the road
> then."_

It nevertheless **doesn't** mean you can't make a general description with
～ていい. It's free to express a general idea taking the form of commenting about
a particular instance, but as long as you do so, you have to pretend it
through. It's like we could say `Does a wombat bite?` instead of `Do wombats
bite?`, but never `*Do a wombat bite?` (nor `*Does wombats bite?`).

_**Acknowledgement**_ : Thanks to @DariusJahandarie for his help and advice
improving this answer.

