I have a dual language news website. I have found a number of 404 errors in example.com which URLs should be in en.example.com, these are actually English content but showing in example.com (Bengali) for this reason 404 errors found in Google Search Console/Webmaster Tools.
This my first experience from Google Search Console. At this time, how will I do to fix the problem?
I suspect you are using relative URLs instead of absolute URLs for example.com. For example, if you are trying to link to en.example.com/page-is-in-here, make sure that your link (or Navigation Menu Link) from example.com is like
<a href="http://en.example.com/page-is-in-here">Go to en.example.com page</a> - Absolute
and not
<a href="/page-is-in-here">Go to en.example.com page</a> - Relative
In the search console reports for example.com, Google believe that /page-is-in-here is actually example.com/page-is-in-here and that is probably why you are getting the 404 errors.
Another probably reason is that when you switch language make sure the requested URLs are absolute and not relative, if you are doing this via JavaScript, make sure that the script is generating absolute url.
I have found a number of 404 errors in example.com which URLs should be in en.example.com, ...
If this is a 100% fact in your situation, then you need to implement permanent redirects. You can set up automatic redirects with your server software to automatically redirect guests from example.com/englishpage/whatever to en.example.com/englishpage/whatever.
Just ensure that when you are done, you try accessing your old link in webpagetest.org or similar tool to ensure the redirect is in place. From a web browser's point of view, the redirect page will normally have HTTP code 301 which stands for "Moved Permanently".
If there are URLs that just aren't supposed to be there anymore on your server, then use the HTTP 410 status code to represent "Gone" so search engines don't keep scanning the page.
As for the remaining HTTP 404 error pages, just fix the URLs so there is content.
