Primary and Foreign Keys

I'm pretty new to databases and sql and i've been having some trouble figuring out the keys. I have certain schema I need to work with and think the primary keys would be EID and DID. However I'm having some trouble figuring out what the proper foreign keys are. Can anyone help me understand this concept. Thanks

By the way I'm working with this schema.
Emp(eid:integer primary key, ename:string, age:integer, salary:integer)
Dept(did:integer primary key, butget:integer, managerid:integer)
Works(eid:integer primary key, did:integer primary key, work_time:integer)

primary key is what makes rows unique

a table can have only one primary key, but it can be a composite

so the primary key of Works would be (eid, did)

each of them is a foreign key to its respective parent table

also, managerid in Dept is a foreign key to Emp


rudy