Most harsh (depending on the size of the mistake):
inattentive (harshness depends on context)
You can also add "a bit" before those words to soften the adjective. Alternatively, you can add "so" before those words to harshen the description.
"She's so ditsy, always bumping into things"
"His writing was a bit clumsy"
"I don't want him at my party, he's so careless when she drinks"
"The parents were so negligent that they had their children taken away."
"An inattentive driver is likely to have an accident"
Unfortunately I have no knowledge of Japanese and can't really properly check the online translation, to be more accurate... however! this sounds like somebody who is clumsy! Clumsiness is the noun, and the adjective is clumsy. It's a very common word. So a person who knocks things over and breaks things all the time is often called a clumsy person, or for example, "my best friend Tom is a great guy, but he's a bit clumsy sometimes!". Some synonyms for the word clumsy are: silly, awkward, uncoordinated, graceless, etc. Another common word for clumsy which is more of a colloquial standard is ditzy! This is also commonly used.
To conclude, clumsy is the word I believe you're looking for :D
EDIT: Oops: "And what do you think what can we do for avoiding making the small mistakes?"
Well, I suppose an example of something one can do is to become more responsible. One can also think carefully, and slow down. Perhaps if one is "forgetfully clumsy", then we can write things down in a diary, so as to not forget. One could also get more sleep! Clumsiness is also a symptom of a possibly more severe issue, possibly due to a neurological problem, in which case; one would have to see a doctor!
I hope I helped!
Hi! I think I would call them careless, inattentive, or negligent. In life, I think we need to be more careful, to think and rethink our actions, in order to avoid careless mistakes.