This lesson covers the subordinating conjunction, when and while. The pattern "當...的時候/dang1...de5 shi2hou4" is used in Mandarin. You can see the same pattern goes on in different sentences that mean different things but other grammatical functions such as progressive 在zai4 and 一直 yi4zhi2 + Verb, are added to elaborate what speaker intends to express.
Near the end of the video is a simple update of my recent school life: Mahjong(麻將) it is! :D Just kidding la!