posts after first out of alignment

So the first post on my site is where it should be: under the header and next to the left sidebar. After that, the posts (the posts themselves, NOT the alignment of the text within the posts) start moving farther and farther to the left of the page. The second one on the page is aligned with the left sidebar, and the ones under that are right up against the left side of the page. Can someone take a look at this and tell me if there’s a reason why only my first post would follow the code?

http://pastebin.com/15Pxi5CE