In contrast to what many people think, teaching your own language as a second language is not easy. Language is present in people´s lives in such a pervasive way that they frequently conclude that it works in an obvious way. Rules are logical and their exceptions are consequently logical too.
If you have worked as a language teacher you have probably noticed that things are not that obvious for your students and, more than once, they surely have asked questions you did not have answers to. If you haven´t done the experiment, well, you will have to trust me: language affairs are neither obvious nor simple.
Learning a language as you did when you were a child will never happen again. For this reason, we have grammar to try to bridge the gap between you and your chosen language.
However, grammar is logical to a fault, since it is always an abstraction of what really happens when people speak.
One of the most common mistakes is to use some simple and even logical rules that are useful to explain a few clear examples but exclude many others. Being simple makes these rules easy to remember and hard to change for more appropriate ones. Therefore, lots of mistakes fossilize in the mind of the students and it becomes a very complicated problem to correct.
The 3 most common examples of these logical but false rules:
1- Ser is used for permanent states and estar for temporary states. This rule does not explain why we say: Soy estudiante, está muerto, la fiesta es en mi casa, and many others.
2- Imperfecto is used for descriptions and Preterite, for one-time actions. The fallacious idea that Imperfecto is for longer periods than Preterite underlies this claim. Exceptions for this rule are: Juan no fue muy simpático ayer cuando hablamos; ayer te llamé todo el día, durante mi adolescencia salí muchísimo, etc.
3- It is impossible to list all the exceptions for the following rule: Subjunctive is used to express doubt, wishes or irreality.
Although it is true that as a teacher, you cannot show grammar in all its complexity, with all its nuances at once, what we can do is to dose out the knowledge throughout the course without instilling in the student the belief that things are simpler than they are. A language is a complex entity and one needs time to grasp it all.
Moral of the story: Don´t trust rules that seem to explain everything and… be patient.