Learn English – the difference in meaning between “Are you for real” and “Are you of real”

It depends on a context. They both may mean the same thing in example like this -

He can hardly write = He can barely write

But then, if you check all the meanings for both the words, you may come up with an example where the words cannot be interchanged.

Now, when in case where hardly refers to 'almost never', replacing it with barely may not sound natural.

"I hardly go for a social gathering" over "I barely go for..."

Edit after the question is edited:

Yes, I agree with djechilin (+1), use poorly or badly instead. The book is poorly translated.

In many contexts, the meanings are pretty much the same, but you might hear finished more often than completed in casual conversation. For example:

I've finished my shopping.
She finished the song.
He finished the race.

I could use completed in those sentences – the meaning wouldn't change, but the register might sound off.

The word completed can convey some sense of accomplishment. In the context of a race, it might work when the race is a major achievement:

He completed his first marathon last year.

Homework, though, is not really a major achievement, so I think you'd hear finished more often in casual conversation:

“Joey, where are you going? Did you finish your homework?”

That said, you might see completed in more formal contexts, such as a paper on education, or a course syllabus:

Students must complete six homework assignments during the semester.