The problem is that when you copy text from Word and then paste it into WordPress, a bunch of extra HTML code gets inserted into the text. You won't be able to see the extra code in the WordPress visual editor, but if you switch to the WordPress HTML editor and know a bit of HTML, you'll notice lots of extra code throughout your blog post that has no reason to be there other than to cause formatting problems on your blog.
Fortunately, there is a way to copy and paste text from Word to WordPress without extra code mysteriously appearing.
Your first option is to copy the text from Word as you normally would then go to the post editor in your WordPress dashboard. Click your mouse where you want to insert the text and the select the Insert from Word icon in the toolbar above the post editor. It looks like a W. If it's not visible, hover over the Kitchen Sink icon in the toolbar and click it to reveal all of the hidden icons. When you click on the Word icon, a dialogue box opens where you can paste your text from Word. Click the OK button and the text will automatically insert into your blog post editor without all of the extraneous code.
The above solution works, but it's not perfect. There can still be formatting issues when you paste text using the Insert from Word tool in WordPress. If you want to ensure there is absolutely no extra code or formatting problems, then the best option is to paste the text from Word without any formatting of any kind applied to it. That means you need to paste plain text, which requires a couple of extra steps, which are explained in the next paragraph.
Simply open Notepad on your PC (or Text Editor on your Mac) and paste the text from Word into a new Notepad (or Text Editor) file. Copy the text from Notepad (or Text Editor) and paste it into the WordPress post editor. No extra code will be added. However, if there was any formatting in the original text that you want to use in your blog post or page (such as bold, links, and so on), you will need to add those from within WordPress.
Another option is to use an offline blog editor to create and publish posts and pages to your WordPress blog. When you copy and paste text from Word to an offline blog editor, the problem with extra code being added usually does not occur and most formatting is retained correctly.