When the verb is made up of one verb character and one noun character. Many verbs in Chinese are actually made up of one verb and one noun. 留言 --> 留 means to leave,言 means words 簽名 --> 簽 means to sign, 名 means name 吃飯 --> 吃 means to eat, 飯 means food 跑步 --> 跑 means to run, 步 means steps 上船 --> 上 means to get on, 船 means boat The other verbs are not like this. Pay attention when reading and you'll get the hang of it! Laurence

As a native speaker and I've learned Chinese for so many years, I've never heard that "break the word and insert something inside." In Chinese, one character is one word. EVERY SINGLE CHARACTER IS A WORD. The "two-character word" is just combine two characters together that can be explained the meaning into those two separate characters. It's not a smart way to learn Chinese as to think "two-character word" is A WORD. That will create thousands of thousands more words to remember and get confused. And if people learn Chinese in "two-character word" or "three/four-character word" way, they can never able to read the novels. The most beautiful and creative thing in Chinese is the writers can combine any two/three/four-character words with any single character and readers still able to understand. Because every single character has it's own meaning. Just to understand them one by one then you get the whole meaning. --------------------- It's free to put the pass tense 了 after a verb or a noun. It both work just has a little bit different meanings. 收到 (to receive) --> 收到了 收 is verb. 到 is a verb too, 收到 both means arrive. They are synonyms. Actually u can say 收了/到了/收到了. 忘記 (to forget) --> 忘記了 忘 is verb. 記 is also a verb. So, 忘記了. In this case, 忘 and 記 are antonyms. 忘 modified 記. It means "not remember". So, 忘 is the main word, then u say 忘了. 簽名 (to sign) --> 簽了名 簽 is a verb. 名 is a noun. So, 簽了名。 u can also say 签名了。 留言 (to leave a message) --> 留了言 留 is a verb. 言 is a noun. So, 留了言 u can also say 留言了。 If the 了 is put after the verb, it modified the verb is finished and emphasize the noun behind. For example: 伤了心， 失了恋， 看了书，洗了脸， 玩了游戏，说了话， 睡了觉，想了她，化了妆，买了衣服，跳了舞，骂了人，写了书，穿了鞋子 If the 了 is put after the noun, it the environment/statement/situation. For example: 伤心了， 失恋了，看书了，洗脸了，玩游戏了，说话了，睡觉了，想她了，化妆了，买衣服了，跳舞了，骂人了，写书了，穿鞋子了 And so on...

“verb + 了”， that means the action has been done. for example: ""我知道了答案"。 i have known the answer. "我知道答案"，that means "i have known the answer" OR "I (always) know the answer". which should be used, that depends the background you speaking.

Laurence‘s answer is very good.

Only when a verb is VO compound. 签(V)名(O)，留(V)言(O)，then you can use V了O to indicate the V actualized in the past, like the past tense. 收到 and 忘记 are not VO, therefore you should plus 了 at the end to indicate past tense.