Impose is a synonym of Enforce. Execute is a type of Enforce. Execute is a synonym of Start. Start is the opposite of Stop. 
Dirt is related to Brown. Brown is related to Nest. Nest is related to Natural. 
Impose <--Synonym--> Enforce <--IsA-- Execute <--Synonym--> Start <--Antonym--> Stop 
Dirt <--RelatedTo--> Brown <--RelatedTo--> Nest <--RelatedTo--> Natural 