How to run queries without double quotes on tables and columns

Sorry, I'm really new to PostgreSQL and am used to Oracle. In function and in regular queries, I usually have to have
doublequotes (") around the names of columns and table names. Otherwise, I get an error that the schema, table, or
columndoesn't exist. I don't always have to though. Sometimes the function works just fine without them even though I
amworking with the same table and schema. What am I doing wrong? 

Thank you,
Susan