TwentyEleven horizontal submenu

You can try positioning the UL absolutely and give a distance from “top”, eg:

#access ul li > ul {
  position:absolute;
}

#access ul li:hover > ul {
  top: 2em; /*This needs to be enough to push it below the main menu */
}

You might also need to float the LIs within that left, or display them inline, eg:

#access ul li:hover > ul li {
  float:left;
}

or

#access ul li:hover > ul li {
  display:inline;
}

Without seeing your HTML and CSS it’s hard to help, but I hope this helps to get you on the right path.