01 - Why my Grass has dried areas
Your grass can have dried areas because of many reasons. One of the most common reasons is water availability from an inefficient watering system. Please check if the dried/patchy area is dried. If is dried then improved your irrigation system.
02 - Why my grass is yellow
Your grass is yellow may be from the lack of feeding or from over-watering. Try feed it with high Nitrogen fertilizers with micronutrients.