1. Explain it simply
快要 … 了 says something is about to happen very soon.
Put 快要 before the verb and 了 at the end: 快要 + verb + 了.
The final 了 is required - say 快要下雨了, not 快要下雨.
You can also say 要 … 了 with the same meaning.
2. Show examples
快要下雨了。
Kuàiyào xià yǔ le.
It's about to rain.
快要 + verb + 了
火车快要开了。
Huǒchē kuàiyào kāi le.
The train is about to leave.
very soon
电影快要开始了。
Diànyǐng kuàiyào kāishǐ le.
The movie is about to start.
final 了 required
3. Share a tip
Keep 快要 right before the verb and never drop the final 了 - it signals the change is coming.