Meaning of 上 as Result Complement I look up in youdao and it says meanings of 上 as below :

(1) from the high position looking down: 登上山顶, 爬上河堤

(2) presents the result of the action: 穿上外衣, 没吃上饭

(3) presents the beginning and continuation of the action: 她爱上了草原。or 他拿起一把铲就干上了。


So, How can I know when to use 上 after a verb? 

Because after reading the definition, I am still confused: to express 1st meaning, used verbs must show the upward direction of action? is 2nd meaning the same as "完"?
from the high position looking down

When the verb denotes upward action or direction, and the object denotes 'comparatively high position', '上' means 'up to (successfully)'

Example:

登山顶 - reach the mountain top (登 shows you are attempting to reach the mountain top, but doesn't clearly indicate you have done so successfully)

登(上)山顶 - reach (up to) the mountain top (登上 indicates you not only attempted, but also successfully reached up to the mountain top)

爬河堤 - Climb the river embankment (爬 shows you are attempting to climb an embankment)

爬(上)河堤 - Climb (up to) the river embankment (爬上 indicates you not only attempted, but also successfully climbed up to the embankment)




presents the result of the action:

When the verb denotes 'applying',  '上' denotes 'onto successfully'

Example:

穿外衣 - wear jacket (the action 穿 must be applied to a body)

穿(上)外衣 - wear jacket onto (上 shows the verb 穿 is successfully applied onto a body)

寫四個字 - write four words (the action 寫 must be applied to a surface)

寫(上)四個字 - write four words onto (上 shows the verb 寫 is successfully applied on to a surface)




presents the beginning and continuation of the action:

when the verb implies it is an ongoing action

Example:

她爱草原 - she loves grassland (the action love is a continuous action. )

她爱上了草原 - she fell in love with grassland (上 show the beginning of the verb 爱, and the verb 爱 itself is a continuous action)



吃上饭 is not an example for 'verb that indicates successfully apply'. It is an example for 'verb that indicate successfully acquire'


吃饭 - eat meal

吃(上)饭 - (successfully) eat (acquired) meal