Перевод - Испанский-Английский - Soy profesor de Química, y les voy a explicar el...

1) "(...) and I'll explain you (...)";
2)"You HAVE TO take AN exam about that subject.";
3)"THANK YOU for your attention."

EXPLANATIONS:

a)e.x.: "I'll explain it TO you if you want..." but "I'll explain you what the matter is if you want...";

b)e.x.: "I must read this book!" means "I'm willing to read this book because I want it, it's my own decision" (someone may have recommended it to me and it made me eager about reading it) BUT "Oh no... I have to read this book for tomorrow..." - "I'm going to read it because I was made to do so, for example, by a teacher" (it's a forced action);

c)"Thanks" is informal.

translation of verb tenses is not all correct it could be