A sentence can be classified as simple, complex, compound or compound-complex. A complex sentence includes one independent clause (a complete clause that can stand alone) and one or more dependent clauses (a clause that needs from another to be complete).
In the case of the sentence presented, "you plan to fail" is an independent clause because it expresses a complete idea; while the clause "if you fail to plan" is dependent because the use of the subordinating conjunction "if" suggest there is a consequence or result and therefore another clause is needed to explain this. Thus, this sentence contains one dependent clause followed by one independent clause and therefore, it can be classified as a complex sentence.
