とき versus 〜たら

A highly common stumbling block for learners transitioning from beginner to intermediate Japanese.

The critical difference is that ～たら assumes that the prior action has been completed before the subsequent action begins. Accordingly, a more accurate translation of ～たら would arguably be ‘once’ or ‘after’.

仕事しごとが終おわったら有楽町ゆうらくちょうに行いく。(✔️)

Once work has finished I’ll go to Yurakucho.

In other words, we can’t start going to Yurakucho whilst we’re still working. One action needs to finish before the other starts. とき, on the other hand, is more flexible in this respect, although for actions that are naturally sequential ～たら is usually the better choice.

仕事しごとが終おわるときに、有楽町ゆうらくちょうに行いく。(△)

When work has finished I will go to Yurakucho.

Note also that the tense of the first clause does depend on the relative timing of the action (as it does in English).

仕事しごとが終おわったときに、有楽町ゆうらくちょうに行いった。(△)

When work finished I went to Yurakucho.

The “Discovery” たら

For other sentences the difference is less distinct, but the nuance changes.

駅えきに着ついたら雨あめが降ふっていた。(✔️)

Once I arrived at the station, it was raining.

駅えきに着ついたときに、雨あめが降ふっていた。(✔️)

When I arrived at the station, it was raining.

Both sentences are acceptable here because we don’t know when it started to rain, anyway. However, Japanese speakers strongly prefer ～たら in this situation. This is due to the “Rule of Discovery”. When the second clause is an unexpected state of affairs outside the speaker’s control, ～たら naturally conveys a sense of surprise (“I arrived at the station and—surprise!—discovered it was raining”).

Sequential vs. Simultaneous Actions

At other times the meaning changes substantially between sequential actions and simultaneous actions.

お父とうさんは新聞しんぶんを読よんだら、ラジオを聴きく。

Once my father has read the newspaper, he will listen to the radio.

Because of ～たら, these actions must be sequential. He finishes the newspaper completely, puts it down, and then turns on the radio.

お父とうさんは新聞しんぶんを読よむときに、ラジオを聴きく。

When my father reads the newspaper, he listens to the radio.

Because of とき, these actions can be simultaneous. He is listening to the radio while actively reading the newspaper.

Uncertainty: “If” vs. “When”

Finally, there is a strict rule regarding hypothetical situations. ～たら can be used for uncertain future conditions (“If X happens…”), but とき can only be used for certain, scheduled temporal conditions (“When X happens…”).

明日あした、雨あめが降ふるとき、うちにいます。(✘)

明日あした、雨あめが降ふったら、うちにいます。(✔️)

If it rains tomorrow, I will stay at home.

You cannot use とき here because you cannot perfectly predict the weather. Because the rain is uncertain, you must use the conditional ～たら.