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I found this form I'm not sure about:

> 少なくとも最後の最後、ばあさんはいいやつに会ったんだ。それで、きもちのいい運転でドライブを楽しんだ。車に乗ってるあいだは、 **楽しかったろう**

The character is speaking about an old woman who, after taking a taxi, killed
herself; the recipient of this sentence is that taxi's driver.

Does this 「たろう」 have the same meaning (or similiar) as 「だろう」? To be sure I I
checked if `past + だろう` (「ただろう」) is a thing, and according to
[this](https://japanesetest4you.com/flashcard/jlpt-n5-grammar-%E3%81%A0%E3%82%8D%E3%81%86-darou/)
page it is; I found
[this](https://japanese.stackexchange.com/questions/29356/for-what-purposes-
is-%E3%81%9F%E3%82%8D%E3%81%86-used) question about a similar form, but I
don't think is the same, since to me my example seems clearly a `past + ろう`
structure, while that case isn't.

(I didn't read accurately all the japanese citation in that answer, since I
was having trouble understanding them; sorry if the answer is already in
there.)

Edit: I just found [this
question](https://japanese.stackexchange.com/questions/3175/colloquial-
contraction-of-%E3%81%A0%E3%82%8D%E3%81%86-into-%E3%82%8D-after-past-tense-
verb-e-g-%E8%A8%80%E3%81%A3%E3%81%9F%E3%82%8D), it does seem to be the same
case.
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question
grammar contractions
Is 「short past + ろう」 like 「だろう」?
> 少なくとも最後の最後、ばあさんはいいやつに会ったんだ。それで、きもちのいい運転でドライブを楽しんだ。車に乗ってるあいだは、楽しかったろう。

「ろ/ろう」 is an informal/colloquial sentence-ender expressing conjecture. It is
used mostly by male speakers (and it does in fact sound quite masculine).

The phrase 「 **ばあさん** はいい **やつ** に会った **んだ** 」 is already quite colloquial and
masculine, so 「ろ/ろう」 fits right in.

「ろ/ろう」 can be replaced by 「だろう」 or 「ことだろう」 for the basic meaning, but it will
surely raise the formality level so I would not call those 'interchangeable'
exactly.

In my personal experience, if that counts, I hear this 「ろ/ろう」 more often in
Tokyo than in Nagoya -- the two cities I have so far lived my life in.
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answer
明鏡国語辞典 第二版 has a dedicated entry for this たろう:

> ### たろ‐・う
>
> ① 過去の事柄や完了した事柄についての推量を表す。～ただろう。「寒かっ━ね」「知っていれば来なかっ━」
>
>
> ②《多く上昇調のイントネーションを伴って》過去の事柄や完了した事柄について相手が同意することを期待しながら確認する意を表す。～ただろう。「君に見せ━？」「おもしろかっ━？」「僕が入院したことがあっ━、高校生のころ」
>
> 過去の助動詞「た」の未然形＋推量の助動詞「う」。
>
> 活用語の連用形に付く。ガ・ナ・バ・マ行の五段動詞に付くときは濁音化して「だろう」となる。「君はもうこの小説を読んだろう」

Etymologically, it's analyzed as たろ + う rather than た + ろ(う). たろ is the
"irrealis" (未然形) form of past-た (yes た can conjugate), and う is the particle
for volition/inference. That said, たろう works as a set, and that's why this
entry exists.

たろう is not particularly dialectal, but tends to sound old-fashioned if used in
essays or novels (it's not archaic but smells like early-Showa or Taisho to
me). It is still sometimes used in masculine colloquial speech today, in which
case たろう is often contracted to たろ, and typically used with an accusatory
overtone (for example 前にも言ったろ! = I said it before!).
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