How to link html pages in same or different folders?

Within the same folder, just use the file name:

<a href="https://stackoverflow.com/questions/908765/thefile.html">my link</a>

Within the parent folder’s directory:

<a href="https://stackoverflow.com/questions/908765/thefile.html">my link</a>

Within a sub-directory:

<a href="https://stackoverflow.com/questions/908765/subdir/thefile.html">my link</a>