http://rtfm.modx.com/display/ADDON/Articles.Theming+Articles
The format of the listings of posts displayed on the main Articles page is controlled by a chunk tpl used by getResources. It's a little bit more complicated since Articles also uses the getPages snippet, which is a wrapper for getResources to provide for paging of long lists of post summaries.
http://rtfm.modx.com/display/ADDON/getResources
http://rtfm.modx.com/display/ADDON/getPage
What you want to do to change that listing tpl is to create a new chunk tpl.
http://rtfm.modx.com/display/revolution20/Chunks
Go to the Templates tab of your Articles resource and see what chunk it says it's using in the Article Row Chunk field. Now open that chunk and copy/paste its content into your new chunk, and change the chunk name in the Article Row Chunk field to the name of your new chunk. The site shouldn't change at all at this point, since your new chunk tpl is the same as the sample one.
Now, open your new chunk and edit it to have the structure you want each row of post summaries to have.
Articles will automatically change the settings for the getRows and getResources snippets that it uses in the background.
If you're careful to keep the same ID and classname attributes in your chunk tpl you won't have to change the CSS, but if you do make radical changes in the tpl you'll need to edit the Article's CSS to properly style your summary rows.
Don't just edit the existing sample chunk, as that will be replaced any time you upgrade Articles. Create a new one and tell Articles to use your new one instead.
[ed. note: sottwell last edited this post 12 years, 2 months ago.]