とき, or
時（とき） in kanji, is a noun that is regularly used to signify ‘when’ something was done. However, unlike some other time expressions,
とき is mainly used to express things that happen at regular intervals (habits, hobbies, etc.), or at broad times/individual events (when it rains, when there is a sale at the shop, etc.)
Using
とき is quite easy! You just need to attach it to the end of any independent word (verbs, nouns, or adjectives). As with most cases where you will attach a noun to another word in Japanese, we will also be required to use な before
な-Adjectives, and
の before nouns.
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狭（せま）い駐車場（ちゅうしゃじょう）で車（くるま）のドアを開（あけ）けたときにドアを壁（かべ）にぶつけた。
When I opened my car door in a small parking lot, I slammed it on the wall. (Individual event)
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暑（あつ）い時（とき）はエアコンを点（つ）けましょう。
When it is hot, let's turn on the air conditioner. (Broad time)
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彼（かれ）は暇（ひま）なときに本（ほん）を読（よ）みます。
He reads books when he is bored. (Hobby)
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雨（あめ）の時（とき）は家（いえ）でゴロゴロしています。
When it rains, I laze around in my house. (Habit)
As discussed previously,
とき is primarily used to discuss broad times. What this means is that it will not be used to discuss very specific events. For example ‘the time you tripped over a log and hurt your foot’. Although this type of sentence will not be grammatically incorrect, it will sound quite unnatural.
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昨日（きのう）、学校（がっこう）から家（いえ）に帰（かえ）ってた時（とき）に木（き）の根（ね）っこにつまづいて、足（あし）を痛（いた）めた。
Yesterday, when I was going home from school, I tripped on a tree root and hurt my foot. (Too specific, another grammar point will work better)
In cases like the above example, using
たら or
て will be much more natural. If
とき is used, the nuance will be that it is describing the whole situation, rather than the more specific event that happened.
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俺（おれ）が高校生（こうこうせい）の時（とき）に俺（おれ）が好（す）きな女（おんな）の子（こ）に告白（こくはく）して、「え、無理（むり）...」って言（い）われた。
When I was in highschool I asked out a girl I liked, and she said ‘umm, no way…’ (時（とき） is describing the broad event of ‘being a highschool student’, while て is describing a single event that happened during that time.