了/le is a temporal modifier.
Therefore, the most common use of 了 is to express completion of an action. Some learners mistakenly believe it is only used for past tense. It is often used for this purpose, however, not all past tense sentences will utilize this character. However, there is more to this character of 了.
Here is a typical pattern including the use of 了：
Subject + Verb + 了 + Object
wǒ mǎi le liǎng píng shuǐ
I bought two bottles of water.
The best way to think of 了 is not as a past tense marker but as a perfective aspect marker.
On the perfective aspect of 了：
The perfective aspect particle le了 marks the perfective state of an action. It indicates that the action is completed. The aspect particle le了 is placed immediately after the verb. It is also called the verb-suffix –le了 or verb-le了. (1)
Another use of 了 can be to mark something that will take place in the future.
míng tiān wǒ shàng bān le jiù gěi nǐ fā e-mail.
Tomorrow I will send you the email.
Hopefully this will help to break down some of the confusion regarding 了. Personally, learning grammar to a point proved useful. However, using Mandarin in the real world made it far clearer and made the learning even more enjoyable. I recommend, if you get the chance to speak with a Chinese speaker, listening for 了 or finding a way to use it yourself!
1. "The Perfective -le Versus the Modal Particle le." Yale University Center for Language Study -- Comet: Course Materials and Exercise Templates. N.p., n.d. Web. 5 Nov. 2012. <http://comet.cls.yale.edu/mandarin/content/Lele/grammar/lele-2.htm>.