There should be more flexibility in using/not using 的 - in most conversation it is dropped when the meaning is clear, such as in this sentence.
You know Mandarin well!
"the" is not needed
You are right
Is it wrong to say 很难完成? I thought it would be a valid sentence meaning they are very hard to complete
I don't think you can grammatically put the words in that order in Chinese.
Hard should also work