Hi Kevin, “了” is a dynamic auxiliary word. The usage of “了” is linked to the tense, verbs and context. I will give you some examples about the usage of “了”as follows： 1. To describe a situation has happened：“了” is commonly used at the end of the sentence here. Sometimes it is a mood auxiliary word in grammar . (1) 我去商店了。 I went to the store. (2) 他去上海出差了。 He was on business in Shanghai. 2. To describe an action is finished：“了” is commonly used after a verb. (1) 汤姆买了一本书。 Tom bought a book. (2) 他写完了作业。 He finished his homework. “以前” means before or former in English. It is mainly used after the subject. I think it is easier to understand than “了”。 For examples： 1. 你以前在哪里工作？ Where did you work before? 2. 我们以前不认识。 We did not know each other before. I hope these answers can be helpful for you to understand these phrases. Anna ^_^

There is nothing same between 了 and 以前，as a matter of fact , there is no english word means 了，it is just a modal particle. http://wenku.baidu.com/link?url=JeCIWoi2ZDiAoBF7Yev0zw-cBL-a-wihGD7nvevFvg_kxUT3SRhuQ1ABNn7L1plsK_sDiImcmO-IOQRDkAWGrtVo8P8qvOX3RMC_mKplGb3

you can't use ''了'in the middle of a sentence ,'了'just a final particles，it's always used end of a sentence to express finished ."以前"is adverbials of time。when we use '以前'it must happened in old time .

了 = expresses past action or condition 以前 = before

了 shows that the action you're talking about is completed. This could be already completed (in the past) or to be completed (like when you say "I'll go to the movies when I will have washed the dishes.") 以前 just means "before" or "previously". It marks past time. 了 does NOT have to mark past time. Chinese has aspect (particles that show whether the verb action is completed, habitual, ongoing, etc.) instead of tense (endings showing when a verb's action occurs).