The construction and etymology of -たら


Ever since I learned that "if" and "when" can be used as translations for the
-たら conditional form depending on the context, I have been looking for an
explanation of its construction in terms of its etymology to have a proper
understanding of how it works. The sources I have looked up say that it is
formed from the "past tense" -た form + ら, but I have failed to find any
information on this ら. Is it a classical Japanese auxiliary verb? Is the full
form -たらば? Does it have anything to do with the 連体形 or attributive form of
classical verb ending ～たり? Or is -たら the 仮定形 form of -た?

Please enlighten me on this topic so that I can stop translating this
expression to English and start seeing it in terms of Japanese.




As regards the grammar of the modern Japanese, it is 仮定形1 of
[た](https://dictionary.goo.ne.jp/word/%E3%81%9F/#jn-132183), auxiliary verb
meaning past tense. At the same time, the standard etymology says た derives
from たり2, so your guess is not totally off.

> ［助動］［たろ｜○｜た｜た｜ **たら 1**｜○］ **《助動詞「たり」の連体形「たる」の音変化
> 2》**活用語の連用形に付く。連用形が撥音便、およびガ行がイ音便となる場合には連濁で「だ」となる。

What is a little irregular about たら are:

  1. It can be used without ば, unlike most other 仮定形: The linked dictionary entry notes 仮定形「たら」は、多く「 **ば」を伴わないで** 「雨が降ったら中止だ」などと使われ、「遅いからもう帰ったら」のように文末に用いられて8の意を表す。

  2. For 仮定形, you may expect e-sound, i.e. たれ. But it retains classical 未然形: [Wikipedia/已然形](https://ja.wikipedia.org/wiki/%E5%B7%B2%E7%84%B6%E5%BD%A2) notes 形容動詞とコピュラ「だ」、「た」だけは仮定条件の「ば」の前が未然形である **文語の形式を残している** ため「なら」、「たら3」になっている。

たら3 refers to the (classical) 未然形 of
[たり](https://www.weblio.jp/content/%E3%81%9F%E3%82%8A):

> ［助動］［ **たら
> 3**｜たり｜たり｜たる｜たれ｜たれ］《完了の助動詞「つ」の連用形に動詞「あり」の付いた「てあり」の音変化》ラ変以外の動詞、および動詞型活用の助動詞の連用形に付く。
>
> [補説] 中世以降は...「たる」を経て現代語の「た」に引き継がれる。

