Impairment. Bereavement is a natural consequence of loss; it allows us to mourn that which we have lost and readjust our world view. Although it makes us sad and may disrupt sleep and concentration for a time, it has a pretty predictable course. Adjustment disorder is a reaction to some sort of situation that causes disturbances in mood and/or behavioral that causes problems for the person and others.
Depends...? So, the rigid definition is...You don't. Basically, that's why we have psychiatrists. That is, doctors who are specifically trained to tell the difference between a problem, and a "not problem." or, so to speak, trouble, and a "normal life" event. The dsm is great...For research. But, human life is simply too subtle and nuanced to lend itself to any such neat and clean categories!