Simplify the following text for a Grade 4 native English-speaking person.
Original: There have in fact been six separate Concorde Agreements , all of whose terms were kept strictly secret : The first in 1981 , others in 1987 , 1992 , 1997 , and 1998 , and the current agreement in 2009 .
Simplified: There have been six Concorde Agreements since 1981 . The terms of all of the agreements have been kept a secret .