.htaccess, permalinks and 500 errors

I’ve tried to search in the forums for my problem and haven’t come up with an exact answer yet. So here is the problem:

I’m using WordPress as a CMS for a business site. To make this happen I’ve utilized the Static Front-Page Plugin and the site is composed primarily of Pages.

I just tried to change the Permalink structure (customizing it to: /%year%/%monthnum%/%postname%/). I had not previously uploaded an .htaccess file to the site’s root. So, after I updated the new Permalink structure in the admin panel, I created an .htaccess file and inserted these rules that wordpress gave me:

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

I then uploaded the file to the root directory (wordpress is my site so it is located /) using ftp. When I went to check the site, I got an Internal Server Error 500. I then deleted the .htaccess file from the server, and when I went back to check my site, only the homepage was accesible. I got 404 errors on any other page. I think this happened because the browser was searching for https://mydomain.com/about-us/ for example and that obviously didn’t exist because the .htaccess file didn’t work.

Anyway, I re-uploaded various wordpress files and changed the permalink structure back to the default and can now access the site’s pages. But my question is, how can I change the permalink structure and upload a valid .htaccess file? Does anyone have any idea why this is happening?

Thanks