と vs. たら for Past Tense Conditionals

Question

When learning about the ~たら conditional, it was presented to me as a conditional which has at least some degree of uncertainty (in contrast with と conditionals). So I was surprised to see it being used in the past tense, like in the following:

ドアを開けたら、加藤さんが立っていました。

When I opened the door, I saw Katou standing there.

Here, "opening the door and seeing Katou" was something that definitely happened, right? So this seems like an exception to the general "~たら conveys some level of uncertainty" rule, no? Or perhaps the "uncertainty" is that it was surprising to see Katou when the door was opened? If that's the case, would it be incorrect to use ~たら in this case if the speaker was for sure expecting to see Katou after opening the door?

In any event, what is the difference between the above conditional and this one:

ドアを開けると、加藤さんが立っていました。

?

Answer 1

(Based on 中上級を教える人のための日本語文法ハンドブック p408)

In Japanese linguistics, 'definitely happened' type of conditional expressions is called 事実的条件. In this type of conditional expressions, only と or たら can be used, and not ば/(の)なら/のだったら.

〇窓を{開けると/開けたら}富士山が見えた
×窓を{開ければ/開けるなら/あけるのだったら}富士山が見えた
Opening the window, I saw Mt. Fuji.

This and the one in question is a subtype 発見, where と and たら can be used interchangeably. That is,

• S1 と/たら, S2

always works if it means When S1, it turns out S2. So both sentences in the questions are natural.

In the cited book, it says

• '後件が前件の動作・出来事の結果生じる無意志的な出来事の場合、「と」「たら」どちらも使える'
• Both と and たら can be used when the consequent is an volition-less event resulting from the action/event of the antecedent.

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FYI:

The other subtypes of 事実的条件 mentioned in the book.

• 前件の動作の結果後件が起こった場合、「たら」は使えるが「と」は使いにくい
• When the consequent is a result of the action in the antecedent, たら is fine but と is rarer.

友達に頼みごとを{〇したら, ?すると}OKしてくれた
When I asked a friend for a favor, he said ok.

• 前件と後件が連続する動作・出来事の場合、「と」は使えるが「たら」は使えない
• When the antecedent and the consequent are successive actions/events, と is fine but たら is impossible.

ドングリはころころと{〇転がると, ×転がったら}池に落ちた
After rolling forward, the acorn fell into the pond.

When the subject is I, と is odd too.

昨日私は家に{?帰ると, ?帰ったら, 〇帰って}すぐ寝た
Yesterday, I went to bed as soon as I got home.

(I changed some examples from the book. The ? is × in the book, for me it is odd but ok).

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1. When the door was opened, the speaker saw Katou (and nothing*/*no one else).
2. When the door was opened, Katou was seen almost as if by a law of nature ("of course Katou was there"...).

To 2, no. To 1, partially yes. As the term 発見 suggests, the construction/usage suggests that the subject 'find out' the consequent. Using an analogy of camera, the sentence is like a camera shooting the speaker doing S1, then S2 comes up from the perspective of the subject (the viewpoint of the camera switches to the subject's eyes). So, it implies what comes to the eye/consciousness of the speaker is mainly Kato. There may or may not be someone else.

This does not change by と or たら.

3. Or some other connotation having to do with conditional "certainty"?

Compared with other usages, if any difference should be noted, the 発見 usage implies simultaneity (S1 and S2 happens almost at the same time).

Answer 2

According to A Handbook of Japanese Grammar Patterns for Teachers and Learners, for this pattern of conditionals where S2 is in the past tense たら tends to be used in conversation and と in writing/novels. This is because と looks at things from a more objective, outside point of view while たら talks about the speaker's own experience.

For both と and たら, the past-tense S2 cannot be a voluntary action of the speaker. It can be used in a few different contexts, but the most common is one of "discovery" like in the sentence you found. (e.g. "When we got to the top of the hill, the enemy army was beneath us")

I think I would frame the certainty/uncertainty difference between と and たら differently. If you want to use those words, I would make it about the certainty of the connection between S1 and S2, not the certainty of S1 (both と and たら can be used to mean "when", e.g. "when it's noon x will happen," as well as if) or the certainty of S2 (as you've noted S2 can be past as well as future).

と tends to be used for general rules, where the connection between S1 and S2 always holds true ("If you drop this, it will fall."). It can be used in other contexts, like this one, but I think that is the paradigm case to keep in your head.

Meanwhile, the wasabi website has a great one-word summary of the expansive たら: たら is a "one-time" conditional. It describes particular cases ("If it's warm today, let's go to the park." not "Whenever it's warm I go to the park."). Thus I suppose you could say it's uncertain in that the connection between S1 and S2 doesn't always hold.

Conditionals are hard. My strategy so far has been to memorize the specific cases where one is obviously more appropriate than the other, and read a lot to hopefully get some sort of instinct for the muddled middle.