Problem with articles in category and subcategory.

Hello.So my problem is the following,I have a parent cateogory(Terra) and then I made a child category(Istoria Pamantului) for it.But i want to put some articles only in the child category and some only in the parent category.But the problem is that even tho i select where every articles should be the ones in the child category appear also in the parent category,and the strange thing is that when i select from the home-page meniu the child category everything seems fine and it only shows the one i put in that category,but when i click on the parent cetegory all of them appear again(parent + child category articles).I use the free version of the blog and update it through the webiste,so I didn’t download anything.This is my blog so you can see what i am talking about(https://fizicaintalnesteconstiinta.wordpress.com/).
If you could help me it would be awsome.Thank you

Adrian