Good question. It's a quite idiomatic expression. With 才 used to express a sense of lateness, they just don't go well together... I know it doesn't logically make sense, but it's like since it happened late, it's kind of not satisfactorily completed. So, don't use 了 with 才.
![4 Most Common Adverbs of Frequency in Mandarin Chinese [Grammar] -- 从来不，有时候，经常，总是](http://i.ytimg.com/vi/cC8OKK3wtVE/mqdefault.jpg)
![4 Most Common Adverbs of Frequency in Mandarin Chinese [Grammar] -- 从来不，有时候，经常，总是](/img/tr.png)
感谢你😍😍
不用谢！
我 只看这个录像一次，我就了解
诶，你这个“就”用得很棒！Very good! 👍 If you allow me to make it even better, you can say "这个视频我只看一次，就明白了。”
請問，為什麼不說：他十點才來了。
Good question. It's a quite idiomatic expression. With 才 used to express a sense of lateness, they just don't go well together... I know it doesn't logically make sense, but it's like since it happened late, it's kind of not satisfactorily completed. So, don't use 了 with 才.
电影八点就开始了但他还没来才八点半
👍 Very nice, just change the second part to "但他八点半才来". 😊
感谢你😍😍
不用谢！