So why not yellow clover?
It looks like a clover. It is called Black Medic.
A little research costs nothing but a little time.
Get your lawn aerated this year.
While I did not start this post as an advertisement, the above link did bring to mind the one process in lawn care that should be included in any lawn care program; core aeration. Speaking of that if you spend money taking care of your lawn; water is essential to a healthy lawn. TURN ON THE SPRINKLER!