I've been wondering which is the most difficult aspect in the Spanish grammar. Spanish has a large assortment of past tenses, irregular verbs, exceptions in the use of some verbs and, specially, the number one hazard called subjunctive. If we dip into the other romance languages, we notice that all of them has a sort of subjunctive. However, in contrast with Spanish, these romance languages don't use it as much as in my mother tongue. In fact, it's almost certainly that if you must discuss about a determined issue in Spanish, you will be nearly forced to use the subjunctive so as to produce some important structures.
The subjunctive is a genuine challenge for the foreigner student. I know foreigners who have been living in Spain for many years and they still have problems when they are obligated to use the subjunctive.