It means like "the time when" As the person before me wrote: "図書館で本を借りる とき、カードが要ります。 = When you borrow the book at the library, you need a card (for the library). " You can also use it in past tense with -た　とき (-ta toki) 日本にいたときに、日本語を勉強した。 =When i was in japan, i studied Japanese. I hope it was enough explanation :)

図書館で本を借りる とき、カードが要ります。 = When you borrow the book at the library, you need a card (for the library). This とき is used for an action related / connected with other action. (I think my English isn't sufficient to explain well about this grammar, please wait better explanations.)

図書館で本を借りる とき、カードが要ります this is the sentence i read ^_^

Hi, do you have example sentence? Is it like "when + subject + verb, ..."? (When I came back to my house, it didn't rain.)