- Por vs. Para (very basically, for vs. for)
- Preterite vs. Imperfect (the two past tenses)
- The Subjunctive (too difficult to explain fully: basically it deals with expressing doubt)
- Ser vs. Estar (the verbs to be vs. to be)
I guess if these four parts of Spanish were easy, the language wouldn't be as much fun. Besides, if it were easy then everyone would be speaking it (less the continent and a half as well as a European country, among other parts of the world).