The Completed Action Aspect Marker 了

In these interesting examples, 了 is used as an aspect marker of the completed action or change of state. When you need to deny that an action has been completed, 了 is omitted. The negative sentence is made by putting 没(méi) before the verb. 

More:

Verbs:

know 知道, 认识, 知, 懂得, 晓得, 了

finish 完, 结束, 完毕, 终, 磨光, 了

understand 理解, 了解, 明白, 懂, 懂得, 了

settle 解决, 结清, 安家落户, 了, 奠定, 收拾

dispose of 卖掉, 了

look afar from a high place 了

Auxiliary verb:

past tense marker 了

Adjective:

clear 明确, 清楚, 明显, 清, 晴, 了