了 can be used in many ways, and it has different functions.

somtiemes it can be used to present the tense.

this article is about illustrating how does the Chinese presents the concept of tense by using the example of 了

for example, you say "I ate a cake yesterday." 

in english, you change eat to ate.

in chinese, we don't change the character, we add a character, 我昨天吃了一块蛋糕。

these two languages have different understanding on simple present tense.

in English, you can say "I eat a cake.",but there is no "我吃一块蛋糕" in Chinese.

it only make people confuse. Because Chinese believe that there are several state:

I ate a cake yesterday. 我昨天吃了一块蛋糕。

i'm eating a cake. 我正在吃一块蛋糕

i' ll eat a cake tomorrow. 我明天将要吃一块蛋糕。

in a world,  English present the tense by inflections, while Chinese present the tense by adding characters.