How can I use someone other's article/post on my own website? What is the legal way of doing this? Usually what I do is that I mention the source from where I got that post. Is there any other way? What ways do you follow to prevent yourself from copyright issues? Can I just use their content and not mention their name? Can I use a hidden HTML link of the source?
Assume that you can't reuse content unless told otherwise.
Unless the page specifically states that the content is licensed for reuse under a Creative Commons licence or similar, or the work is in the public domain, you should assume that content is not licensed for reuse, and not reuse it in any way.
You can also use the Creative Commons search tool to find works that are licensed for reuse.
It's important to respect people's rights on this one; publishing content on the Internet does not transfer any rights to people who can access that content, nor does it put content into the public domain in the legal sense. The copyright automatically belongs to the content's author, be it a person or a company, and they alone can license its reuse.
If you can't see a licence and you want to reuse content, contact the author to ask if it's OK. If they don't reply, assume that you cannot use it. Contacting them does not transfer any rights to you. You need a reply in writing approving reuse.
If you are currently reusing content, and the original author has not provided a licence on the page to do that or otherwise given you permission in writing, you should stop stealing their content. Write your own, hire writers, or use Creative Commons -licensed work instead.
In addition to Nick's answer, if you're going to reproduce the content from another website, you should use canonical URLs referring to the original source so search engines know which content is the original work.
Also, from an seo point of view, reproducing another website's content will have no SEO value to you as it will be filtered out as duplicate content.
Is it mandatory that you use the entire article or post? You should be fine so long you only reproduce part of the content and invite your visitors to read the rest of it in the original website.
It's not a copyright infringiment if you're just mentioning it, think of it as a quote. To avoid losing visitors to the sites you link, it's important you describe the content with your own words and, of course, produce content of your own.