Question
how do i kill grass in flower beds without harming other plants

Answer
This is why God invented Fabric Covers.

Best thing to do -- next Spring -- is to pull up the Weeds you see, then cover with dark landscape Fabric.

Cover the Fabric with 1 to 2 inches of Mulch.

Voila!  Problem solved.

You can get landscape Fabric at Home Depot, Lowes, Target, Kmart and a host of other stores, but it usually has to be in season.  Landscape fabric works beautifully by letting moisture and air through in one direction, but no Weeds or wayward Grass can surface.  It's the greatest thing since sliced bread.

It works all season, and a second season, and maybe 3 or 4 seasons if you're lucky.  The time you save you can play the Sims.  Go biking.  Read a book.  Enjoy your Garden.

Thanks for writing.