とき

1. 1When or at the time of an action; the main clause happens in connection with that action.Ex. 1
2. 2During a state or period of time, such as one's childhood.Ex. 2

とき is a neutral, everyday 'when' used in both speech and writing. The verb form before it affects timing: with 辞書形, the main event typically happens before that action is completed (日本へ行くとき = before arriving in Japan). With た形, the action is treated as having happened or being in progress, so the main event falls during or after it (日本へ行ったとき = once in Japan). It is the basic, all-purpose time expression, while related patterns add condition, formality, or duration.

• 場合はN4「場合は」 is used to set up situations that may possibly occur, is followed by countermeasures or rules, and has a formal tone. It can often replace 「とき」 when talking about possible situations, but it cannot describe a one-time event that actually happened in the past, nor does it express periods like 「子供のとき」.
• 際にN3「際に」 is more formal than 「とき」 and is used mainly in announcements, explanations of procedures, and business settings; it usually follows a verb in dictionary form or た form, or a noun + の. When following a verb or noun it can be replaced with 「とき」, but using 「際に」 in casual chat sounds stiff.
• たらN4「たら」 carries a conditional meaning, expressing hypotheses and unexpected discoveries; its first clause takes the た form or 「だったら」. 「とき」 expresses only plain time and can take the dictionary form to show the first action is not yet complete, and the two are close when saying 'after ...' or 'what to do in a certain state'.
• うちにN3「うちに」 does more than just indicate time: one use is to do something promptly before a state changes (as in 「温かいうちに」), and the other is to describe a change that occurs without one noticing while something is ongoing. To simply say 'when ...' use 「とき」; using 「うちに」 adds this extra nuance.
• 間にN4「間に」 emphasizes a period with a clear beginning and end; the main clause describes something that happens at a point within that period, and it can also express getting something done before the period ends. 「とき」 refers only to a point in time or a period, and both patterns are neutral in register.
• 最中にN3「最中に」 highlights the very moment an action is in progress, and what follows is often an unexpected event that suddenly interrupts it; it attaches only to a noun + の or the 「ている」 form of a verb. Most places where 「最中に」 is used can be replaced with 「とき」, but the nuance of 'of all times' is lost.

Unlike 場合は (in the case that, often hypothetical), たら (conditional or 'once X happens'), and the more formal 際に, とき simply marks the time something occurs. うちに (before a state changes), 間に (within a span), and 最中に (right in the middle of) focus more narrowly on duration or timing within a period.