The controlled vocabulary system for PubMed is MeSH: Medical Subject Headings. Using MeSH in combination with keywords is the most effective way to search PubMed.
For example, here are some MeSH terms for:
Adults: Adult (A person having attained full growth or maturity. Adults are of 19 through 44 years of age. For a person between 19 and 24 years of age, YOUNG ADULT is available.)
Screening: Mass Screening (Organized periodic procedures performed on large groups of people for the purpose of detecting disease.)
Major depression: Depressive Disorder, Major (Marked depression appearing in the involution period and characterized by hallucinations, delusions, paranoia, and agitation.)