Complex sentences are formed of one independent (main) clause, and one or more dependent (subordinate) clauses. The 'sequence of tenses' rule mean that verbs in the subordinate clause must be 'in agreement' with the verb in the independent clause. http://easyenglishtime.blogspot.com/2014/05/the-sequence-of-tenses.html
..by Milena Ristić
When the police found my lost kid I felt so reieved.