啦【la】

啦 is 了 and 啊 said as one syllable. It goes where a sentence-final 了 would go, and adds the excitement, relief or impatience of 啊. It belongs to casual speech and chat messages.

Telling someone to stop

别 + verb + 啦 is a softer, more familiar "stop it" than 别 + verb + 了.

Listing

After each item in a spoken list, 啦 is just 啊 with a different sound, and has no 了 in it. It means "things like ... and ...".

Only at the end

啦 replaces the 了 that ends a sentence. The 了 right after a verb with an object following stays 了.

• × 我买啦三本书 → 我买了三本书
• × 他吃啦饭就走了 → 他吃了饭就走了

In writing and formal speech, use plain 了.