”了“有很多用法，通常在汉语课中，需要很多课时来解释”了“在不同情况下的不同用法。“了” is not only used in the past. 举几个例子：
（1）The modal particle ”了“
This "了" is used at the end of a sentence, indicating the happening of something, the completion of an act or the emergence of a circumstance and the change of a situation.
a. the happening of something: 他去公园了（还没回家）。
b. the completion of an act: 他上午去公园了，现在回家了。
c. the emergence of a circumstance:他突然摔倒了。
d: the change of a situation: 夏天来了，树叶绿了。
try to compare those two sentences:
a: 你去哪儿？(Where are you going?)
b: 你去哪儿了？(Where did you go?)
（2）The situation of ”Verb + 了“：Usually there is one ”了“in a sentence, why are there sometimes two ”了“ in one sentence?
a: If there is an object after "verb + 了", the number-classifier phrase or other word is required before the object.
(a) 我买了一本书。（I bought a book）
(b) 他吃了三个苹果。（He ate three apples）
b. If the object does not have a number-classifier phrase or other attributes before it, "了" must be ALSO added at the end of the sentence to make the sentence complete. The function of such a sentence is to transmit certain information and draw the attention of the listener to a fact.
(a) 我买了书了。（which means “you don't have to buy the book for me because I have bought the book already”）
(b) 他吃了三个苹果了。（which means “He has eaten three apples, can't take more now”）
Today I went to a cafe. - 今天我去咖啡店了。