1. Explain it simply
A 了 at the END of a sentence marks a CHANGE - a new situation.
天气冷了 means the weather has turned cold (it wasn't before).
我会说中文了 means I can speak Chinese now (a new ability).
Keep this 了 at the very end; dropping or moving it loses the change meaning.
2. Show examples
天气冷了。
Tiānqì lěng le.
The weather has turned cold.
new situation
我会说中文了。
Wǒ huì shuō Zhōngwén le.
I can speak Chinese now.
new ability
他饿了。
Tā è le.
He is hungry now.
change of state
3. Share a tip
Sentence-final 了 signals a change; it must stay at the very end, or the 'now/change' meaning disappears.

