Teach us to number our days

What does it mean “Teach us to number our days”?
First, it means that teach us the sense of vanity.  A lot of people have a goal but no purpose.  They know how and where to go but don’t know why.  Why a life is vain? (1) It is because the life goes back to dust [(Ps.90:3) “You turn men back to dust, saying, ‘Return to dust, O sons of men.’”  (2) It is because the life is very short[(vv.4-6) “For a thousand years in your sight are like a day that has just gone by, or like a watch in the night.  You sweep men away in the sleep of death; they are like the new grass of the morning- though in the morning it springs up new, by evening it is dry and withered”].  (3) It is because our life span is but trouble and sorrow [(v.10) “The length of our days is seventy years—or eighty, if we have the strength; yet their span is but trouble and sorrow, for they quickly pass, and we fly away”].  

Second, teach us how to live a valuable life.  How can we live the valuable life?  First, we must fear God [(v.11) “Who knows the power of your anger?  For your wrath is as great as the fear that is due you”].  The whole duty of man is to fear God and keep His commandments (Ec.12:13).  Second, we must be satisfied with God’s love [(v.14) “Satisfy us in the morning with your unfailing love, that we may sing for joy and be glad all our days”].  Third, we must glorify God (v.16).  Fourth, we must live by God’s grace (v.17).