Someone who can be claimed as a dependent is a qualifying child or a qualifying relative of the taxpayer. Someone who can’t be claimed as a dependent is a spouse of the taxpayer, or who is not a U.S citizen.
Example: a mother wants to place her child as a dependent, she has to make sure she qualifies and is the right age. The child is 16, is the child the right age? The answer to that is yes. The child has to be under age 19 or 24 if they are a full-time student. The child qualifies to be a dependent for the mother.
2 example: a woman wants to make her spouse a dependent but she has to make sure he qualifies. The spouse is not a U.S citizen so he cannot be a dependent. She must find someone else who can be.