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.


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


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.

  • 1
    Sorry, I revoked my answer. There seems to be more things to consider. Dec 5, 2014 at 15:12
  • 2
    It's definitely なったら... I couldn't explain why though. On the bright side, once you hear it a million times it won't matter. :) My only advice is to not think of たら as "if" in your head every time, as it definitely doesn't work like that. The sentence above is a great example. Dec 5, 2014 at 15:54
  • The only thing bothering me is that "信号があおになると、道を渡っていい" sounds not so bad to me, while the original "信号があおになると、道を渡っていいです" sounds very bad. How the two acceptable to you guys? Dec 5, 2014 at 17:18
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    @broccoliforest I hardly trust my own judgments here, but as a guess, perhaps it's possible to interpret the former as an ellipsis of 道を渡っていい[ことになる] (or something equivalent), which becomes impossible when the sentence-final polite marker です is there? Dec 5, 2014 at 19:08
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    @DariusJahandarie Thank you, maybe it's the explanation in this case. Though I have a bit more grammatical concerns, I'm going to restore my answer with some updates, because all of those anomalies are virtually if not completely unheard. Dec 5, 2014 at 20:59

2 Answers 2


You can only use なったら, not なると.

First, take a look at this topic, 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.


"なったら" is correct, because "なったら" describes conditions, on the other hand "なると" describes subsequent events.

信号{しんごう}があおに(    )、道{みち}を渡{わた}っていいです。

"いいです" means "allow" or "feel free to do", so the former half of this sentence imposes a condition to be able to cross the road. You can make a choice freely whether you cross the road or not.

"なると" means "When A occurs, B almost always occurs". The latter half of the sentence you give says about something you can do, not about some events which almost always occurs after the traffic light becomes blue. So "なると" is incorrect.

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