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
All the work I've been putting into this project has been ll for nothing because my boss is shutting it down.