Recordar in El Pretérito Imperfecto

Question

In one of the quizes  engish scentence "After the blow, you didn't remember anything" one is asked to conjugate recordar in the El Pretérito Imperfecto tense.  I used "recordaba" but this was marked as incorrect.  recordabas was marked as corrrect. Why is the "You formal" conjugation not correct for the above scentence?

Answer

Recordaba would be correct if they asked for "you" usted (formal). But,  the question was specific to  "you"  Tú (informal.)

Answer

The spanish given was "Después del golpe, no  <select>  nada." , where one had to select for a list of answers.  There is nothing in above spanish that appears to indicate that the statement is addressed to an aquientance vs. stranger in a formal situation. How can one tell the correct conjugation is for "Tú"?  Is there maybe an informal rule of some sort I missed?

Answer

Hi Steve,
You are right. There is nothing telling you whether you need the informal or formal form. I added this to the hint: "Conjugate the "tú" form of "recordar" in El Pretérito Imperfecto.", so it is clear now which one we are asking for. 
Gracias y un saludo,
Inma