The verb forms of founder and flounder are sometimes confused because they have similar connotations. These verbs, however, have two distinctly different meanings.
Founder can be a noun or a verb.
As a noun, a founder is “one who founds,” as in someone who establishes an organization.
Example
As a verb, founder means “to break down” or “to sink.” For example, a foundering ship is one that is filling with water and sinking.
Example
Flounder can also be either a noun or a verb. As a noun, a flounder is a type of fish.
As a verb, flounder can mean
This last definition, “to be close to failing,” is sometimes confused with founder. As you can see, though, the two words’ meanings are not the same.
Something that is floundering is in danger of—but has not yet—foundered.
Example
Hint: Flounder swim; they do not sink.
Link to exercise on founder / flounder.