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What is the difference between 结束和完成？What about 完了？

In many cases, they are interchangeable. 完成 has a sense of achievement and completion. 結束 means the process of something is finished/complete/over.

了 indicates past tense.

完 is a complement like 好, 到 -- -- complements go after verbs.

EX:　吃完了(finished eating it), 做好了(it's done, and done well), 看到了(saw it, achieve the seeing of something)

There all have a same meaning.
'结束' —finish
'完成'—complete
'完了'—over, which exists mostly in the oral.

结束
over, end
game over 游戏结束
they ended the play with a song. 他们以一首歌曲结束了这出剧。

完成
finish, complete
when will you finish your homework?你什么时候完成家庭作业？
when will the building work be complete?建筑工作什么时候完成？


完了 this one is more colloquial, always used in the end of a sentence or alone.
1 gosh
完了，我忘了带钥匙。oh, gosh, I forgot my keys.
2 finished, over
作业做完了 work finished
书看完了 finished reading the book
课上完了 the class is over

finished, the end

1.完了，是指某事，某物，某些抽象的，或者某些具象的东西不再持续，不再继续的意思。
2.结束 表示某些事情，某些关系，通常是指物体，或者抽象事情，表示完了的意思，没有了，不再继续了。
3.完成 表示某些事情Done好了，FINISH。
4.SO. 所以完了>结束>完成

It is kind of done-->結束和完成。 screw up --> 完了。