The beautiful, the good: cái + adjective

Cái in front of an adjective, and sometimes a verb, turns it into the quality itself: cái đẹp (beauty, the beautiful), cái mới (the new), cái ác (evil), cái chết (death). Cái tôi, literally "the I", is the ego, your sense of self.

You'll meet it in two kinds of text. Essays, literature lessons and papers use it for ideas: quan niệm về cái đẹp, a view of beauty; dám đón nhận cái mới, daring to embrace the new. Weather news uses it for what you feel on your skin: cái nóng gay gắt (the fierce heat), cái lạnh đầu mùa (the first cold of the season).

It sits where any noun sits, often with a describing word after it: cái nóng oi bức, cái tôi cá nhân.

Its neighbour sự also makes nouns, but for processes and states that are discussed rather than felt: sự phát triển (development), sự quan tâm (attention). Each word goes with one of them by use, so you write cái đẹp but sự phát triển, not the other way round.

In conversation you'll still hear cái nóng and cái lạnh, but people mostly just describe the thing: Trời nóng quá! rather than talking about the heat, and Đẹp quá! rather than about beauty.