How to determine when it becomes depression?

Interferes with life: A general sadness becomes a clinical depression when the sadness starts to cause problems in the person's relationships and work. Also, if the person has a very low mood, loses interest in things previously enjoyed, has problems sleeping, increased hopelessness, weight loss or gain, and thoughts about death (including suicide but not only) can all be signs that normal sadness has become depression.

Agree w Dr Berman.: What differentiates depressed mood from a major depression is that it causes problems with function in a number of areas of life ; it reduces quality of life.