law and gospel

What is the role of law and gospel in the Christian life? Here's a great hymn written by Isaac Watts in 1709 to help us understand this relationship:

The Law commands and makes us know
What duties to our God we owe;
But ’tis the Gospel must reveal
Where lies our strength to do His will.

The Law discovers guilt and sin
And shows how vile our hearts have been;
The Gospel only can express
Forgiving love and cleansing grace.

What curses doth the Law denounce
Against the man that fails but once!
But in the Gospel Christ appears,
Pard’ning the guilt of numerous years.

My soul, no more attempt to draw
Thy life and comfort from the Law.
Fly to the hope the Gospel gives;
The man that trusts the promise lives.