現代華語裡面「了」主要有五個用法，其中一表示動作的完成；兩個主要表示狀態的改變；另外兩個主要表示某段持續的時間。

❶表示動作的完成：這是最簡單的用法，但也是常常被用錯的文法。因為很多狀況之下「了」可以被省略不用說，但我們依然知道這是過去發生的事情，如：

「我昨天去看電影。」

這個句子應該是很多台灣人都講過的句子，裡面沒有「了」，但因為有「昨天」所以大家都還是知道這是已經發生過的事情。如果依照正確的文法，應該是這樣說：

「我昨天去看了電影。」

或

「我昨天去看電影了。」

可以省略「了」的狀況是：如果是兩個動詞連用。如「去」＋「看」，則通常「了」可以省略。

❷表示狀態的改變：比如從「不會」變成「會」：

「我會打網球了。」

另外還有一個也是類似狀態的變化的用法，但強調的是❸快要發生的變化：

「電影要結束了。」

「要下雨了。」

這邊經常出現的是跟「快」連用。

最後還有兩個跟時間長度相關的用法，❹表示在過去的某段時間持續的動作：

「我上個月在日本東京住了三個星期。」

這邊表示的是過去的某點A開始到過去的某點B結束的時間範圍，並不直接聯繫到「現在」。直接聯繫到現在的是❺的用法：

「小明已經學華語學了五年。」

這句話的意思是從現在開始往前算，小明學華語已經學了五年。也可以說是跟英文”have been Ving”最接近的語法。

In modern Chinese, "了" has five main uses. One indicates the completion of an action; two primarily indicate a change in state; and the other two mainly indicate a certain duration of time.

❶ Indicating the completion of an action: This is the simplest use, but it is also often misused. In many situations, "了" can be omitted, yet we still know it is something that happened in the past. For example:

「我昨天去看電影。」(”I went to see a movie yesterday.”)

This is a sentence that many Taiwanese people have probably said, and it doesn't include "了," but because of the word "yesterday," everyone still knows it is something that has already happened. According to correct grammar, it should be said like this:

「我昨天去看了電影。」(“I went to see a movie yesterday.”)

Or

「我昨天去看電影了。」(“I went to see a movie yesterday.”)

"了" can be omitted in situations where two verbs are used together. For example, "去" (to go) + "看" (to see), usually "了" can be omitted.

❷ Indicating a change in state: For example, changing from "cannot" to "can":

「我會打網球了。」(“I can play tennis now.”)

There is another similar usage indicating a change in state, but it emphasises a change that is about to happen:

❸ 

「電影要結束了。」(“The movie is about to end.”)

「要下雨了。」   ( "It's about to rain.”)

In these sentences, "了" often appears with "快" (about to).

Finally, there are two uses related to the length of time. 

❹ Indicating a continous action happened in a period of time in the past:

「我上個月在日本東京住了三個星期。」

(“I lived in Tokyo, Japan, for three weeks last month.”)

This indicates the time range from a certain point A in the past to a certain point B in the past and does not directly connect to the "present." The use that directly connects to the present is:

❺ 「小明已經學華語學了五年。」(“Xiao Ming has been studying Chinese for five years.”)

This sentence means that from now, looking back, Xiao Ming has been studying Chinese for five years. It is the closest to the English grammar "have been Ving."