Difference in meaning of 場合 with 辞書形 and た形


Most of the grammar sources that I have access to tell me that 場合 can be
preceded by the 辞書形 or the た形, but none of them explain the semantic
difference between the two uses.

This is clearly not a simple matter of time or completion, because the
following sentence is given in みんなの日本語:

> カードをなくした場合は、すぐカード会社に連絡してください。

Here, the losing of the card has not yet happened or been completed. Various
native speakers have assured me that the same sentence would be valid with なくす
instead of なくした but exactly what the difference in meaning would be is not at
all clear to me.

How does the use of the た形 and 辞書形 affect the meaning of 場合 sentences of this
kind?




辞書形+場合 means "In the case someone will do the action". た形+場合 means "In the
case someone has done the action".

I think "カードをなくす場合は、すぐカード会社に連絡してください。" is unnatural because no one will lose
the card deliberately (will have no intention of losing the card). However, if
you often lose the card, you can say like this
"カードを(よく)なくす場合は、カードを財布の中に入れておきましょう。".

For example, in "切符を買う場合、みどりの窓口で買ってください。" and "切符を買った場合、車掌にその切符を見せてください。", 買う
and 買った cannot be exchanged.

