Question
We had friends over a few days ago following torrential rains for a backyard bbq.  We were slipping and sliding on the way to the back yard patio.  This section of the yard is somewhat worn due to foot traffic and when it rains it gets muddy and slippery.  Just an accident waiting to happen.  I want to put flagstones there but what about the spaces in between?  They will still be muddy and soggy.  Will an occasional rainstorm cause permanent damage to grass there and if so how would we minimize that?

Answer
Excess standing water that collects instead of draining in a reasonable amount of time will make it impossible for almost anything to thrive, whether you put down flagstones or not.

Drowning roots are deprived of Oxygen.  Anaerobic bacteria multiply and thrive; plant roots and crowns disintegrate.

This is a common problem, and it occurs in low areas and/or shade.  Muddy and/or compacted soil, slippery rotting grass are definite side effects.  To fix, stop watering or change sprinkler heads to lower volume heads.  Raise low points, or install a drain to move the water somewhere else.  Some serious soil amending will improve tilth and water retention.

Flagstones are not a bad idea.  Feet need a place to land; getting them on the stones is preferable to putting your Grass through foot traffic and injury that will eventually cause irreparable harm.  Let me know what kind of guidance you need on the drainage.  Thanks for writing.