Conditionals in a welcome email: たら, ば, と, なら
Atsushi runs a weekly language exchange at a café near the station. New members often get lost, so he wants to tell them: call me when you arrive at the station. He drafts four versions for the welcome email: 「駅に着いたら電話してください」「駅に着けば電話してください」「駅に着くと電話してください」「駅に着くなら電話してください」. A Japanese friend says two of these sound wrong in an email, one is only natural if the station has already been mentioned in the previous sentence, and only one works everywhere. Atsushi also plans a second line, 「雨が降れば、イベントは中止です」, which his friend calls correct but slightly stiff for a friendly email.
1. Which of the four versions should Atsushi use, and why do the others fail?
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Use 「駅に着いたら電話してください」. たら is the neutral conditional for 'when/if X happens, then do Y', and it freely takes a request in the second clause. 「着けば」 is grammatical in form but ば with a request sounds bookish and unnatural in a friendly email. 「着くと」 is the real error: a と-clause describes an automatic, inevitable result, so it cannot be followed by an invitation or request such as 電話してください. 「着くなら」 is possible but conditions on the idea of arriving, which needs the station to be already under discussion.
2. Explain the と problem in a way Atsushi could reuse when teaching other members.
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The と conditional presents a cause-and-effect pair that happens by itself: 春になると花が咲く, ボタンを押すと切符が出る. Because the speaker presents the result as automatic, the second clause cannot contain the speaker's intention, request, invitation or judgement - 電話してください, 行きましょう, お願いします are all excluded. A useful check for learners: if the second half asks somebody to do something, と is almost always wrong, and たら or なら is the safer choice. This is a rule about meaning, not about politeness level.
3. Show the sentence where 「駅に着くなら」 would be natural, and say what the preceding context must contain.
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Something like 「新宿駅から来るんですね。駅に着くなら、西口にいてください」. なら takes up something already mentioned or assumed and comments on it, so the station or the arrival must have been introduced just before. With that context, なら sounds like 'if that is the case' and is perfectly natural. Without it, the listener wonders which station and why arrival is being treated as a topic. For Atsushi's email, where the station appears for the first time, たら remains the correct choice.