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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。

?
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conditionals
と vs. たら for Past Tense Conditionals
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](https://www.wasabi-jpn.com/japanese-
grammar/how-conditionals-work/#3) 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.
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answer
(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_.

* * *

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).

* * *

>   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).
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