Soy profesor de QuÃmica, y les voy a explicar el tema principal. Ustedes deberán rendir el examen del tema correspondiente. Muchas gracias por su antención.
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);