Strain is a type of Tighten. Tighten is the opposite of Loose. Loose is a synonym of Separate. Divorce is a type of Separate. 
Assure is a synonym of Secure. Secure is similar to Sure. 
Strain --IsA--> Tighten <--Antonym--> Loose <--Synonym--> Separate <--IsA-- Divorce 
Assure <--Synonym--> Secure <--SimilarTo--> Sure 