1. Explain it simply
快 goes at the start, 了 goes at the end: 快 + verb/situation + 了.
It signals something is right on the edge of happening - 'soon' or 'almost'.
2. Show examples
电影快开始了。
Diànyǐng kuài kāishǐ le.
The movie is about to start.
快 + verb + 了
我们快到了。
Wǒmen kuài dào le.
We're almost there.
快到了 is a very common phrase
快下雨了，快点儿回家吧。
Kuài xià yǔ le, kuài diǎnr huí jiā ba.
It's about to rain, let's hurry home.
快…了 describing an imminent event
3. Share a tip
快…了 always needs that final 了 - without it, 快 alone just means 'fast', not 'about to'.