Question: I don't know whether I am missing a trick in HTML. I have one simple HTML website which has over 100 HTML pages. For each page the header, sidebar & footer section are the same but the content is different. Can I simply make this site like Wordpress where the page is formed by header.php, footer.php, page.php & sidebar.php. I know I will still have to work on page.php kind of files. 

This will simplify the file structure of the site. Also I want to know whether this solution is good in SEO perspective or not.

Answer: 
Content Management Systems such as WordPress don't really improve SEO since you can do everything in Static HTML form than you can in a WordPress. 

WordPress just makes thing's a hell of lot easier for managing your pages as well as creating SEO friendly URLS. You should see using a Content Management System as making your job easier and freeing up time to make even more content - If we was to save time saved and time spent on constructing pages then yes it does improve SEO.
Step 1:- Create a index.php in your Website folder and remove index.html.(index.html has preference over index.php)  

Step 2:- Create a header.php and footer.php and place all your code in header and footer.
Step 3:- Now include header.php and footer.php in index.php :- in place of the top header stuff, do the same for footer, head section, etc.
Step 4:- You will need to change extensions of all files to .php from .html.

```
<html> 
<head> 
<title>Page title</title> 
<?php include('header.php'); ?> 
</head> 
<body> 
enter content here
<?php include('footer.php'); ?> 
</body>
</html>

```

Another Approach :-
Step 1 :- Download Twenty Thirteen WordPress Theme and just change header.php and footer.php and sidebar.php.
Step 2 :- Publish pages and copy all html content (excluding header and footer) in the source section of every page.