了 is commonly used for past tense and change of actions. For example, 我哭了 I cried, past tense. 我不哭了 I am not crying anymore, change of action. 的 is used to describe things. 昨天吃的东西, the stuff I ate yesterday. 东西 stuff or thing, 昨天 Yesterday and 吃 eat. 个 is a measure word and measure words are used to say 'a/an' or sometimes plural. Different things use different measure words but translate the same. 个 is the most common one. 一个朋友 a friend 一个人 a person 一个苹果 an apple.三个人 three people 七个朋友 seven friends 五个苹果 five apples.

的 de （someone always spkon di） ① ≈ 's， belong to someone's something 某人的某物 your's 你的 his他的 mine 我的 ② colour (spoken) red 红的 pink粉的 ③adj beautiful 美丽的 right 正确的 dī ① taxi 的士，出租车 dí ①sure，of course ( Express affirmation) 的确

的 is a particle 了 follow the verb or at the end of the setence, shows past tense 个 is a common measure word. Almost every noun has its mearsure word, eg. one person, in chinese we say 一个人（yi ge ren ）。

In most cases,的after the adjective, 美丽的，开心的。了after the statement on or after exclamatory sentence, 我忘记了，我要开学了，个used for quantifiers，一个，两个，几个，比如：我昨天看见了一个美丽的女生。

的 is a symbol of adj or attribute, 的indicates the word or phrase or clause in fron of 的 is adj or attribute, the word behind 的 is a noun; 了is a symbol of a verb or verb phase , 了 indicates the faction of the verb or verb phase in front of 了 is over. 个 is a measure word, for an example , 大的苹果吃了三个，一个小的苹果留给你了。