Could there be an 'invisible' "should" in this sentence? 携帯番号交換しとくんだったな


> 携帯番号交換しとくんだったな。。。

The sentence means "We have interchanged phone numbers in advance." This makes
no sense with the story unless I add "should" before the "have". So could
~teoku's meaning also include "should" ("should prepare in advance") in casual
speech, or something like that, which could have this sentence make more sense
for me?




You are correct.

> 「[携帯番号交換]{けいたいばんごうこうかん}しとくんだったな。。。」

means that the persons involved _**DID NOT**_ exchange cellphone numbers.

「しとくんだったな」＝「しておくんだったな」 = "should have ~~ed"

_**「～～しておくんだった」, which is used idiomatically, expresses one's retrospective
regret of not having done ~~.**_ In this sense, one could call it "invisible"
as it is all idiomatic instead of any part of the phrase literally meaning
"should".

The "visible-'should'" versions are:

「～～しとく **べき** だった」 and 「～～しておく **べき** だった」

Thus, the sentence in question means:

> "We/You/They should have exchanged cellphone numbers."




You are correct.

> 「[携帯番号交換]{けいたいばんごうこうかん}しとくんだったな。。。」

means that the persons involved _**DID NOT**_ exchange cellphone numbers.

「しとくんだったな」＝「しておくんだったな」 = "should have ~~ed"

_**「～～しておくんだった」, which is used idiomatically, expresses one's retrospective
regret of not having done ~~.**_ In this sense, one could call it "invisible"
as it is all idiomatic instead of any part of the phrase literally meaning
"should".

The "visible-'should'" versions are:

「～～しとく **べき** だった」 and 「～～しておく **べき** だった」

Thus, the sentence in question means:

> "We/You/They should have exchanged cellphone numbers."

