This is a common question here on Steemit.
The community values original content. Therefore, you should not ever copy and paste an article from another website unless that website is your own personal blog and the article was originally written by you. There are several projects on Steemit dedicated to removing plagiarized content such as 
Citing sources for things such as images and articles is fine, but again, you're using this platform as a way to potentially earn money, and representing someone else's work as your own for commercial purposes is unethical and generally frowned upon.
If you'd like some more advice on how to create great content for Steemit, check out the Steemit Etiquette Guide and this post I made a while back about polishing your posts.
As a final thought, if you are fascinated by an article elsewhere online and you want to share it with other Steemians, feel free to do a review of it, or share your own thoughts on the subject matter. Feel free to use quotes from the article and cite the source to help make your case. An example of this may be found here, where outside sources are used throughout, but referenced appropriately.
If you have any questions, let me know!
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