How to say these in Japanese?

Question:

1. Let's begin (the class)
2. Let's end the class here
3. I want to say "please repeat?" is "moichido onegaishimasu" correct?
4. What's the differences and When are these words for fire used? 火 and 火事

Answer:

1, (授業を)始めましょう。
2, これで授業を終わります。
3, An accurate translation is like 繰り返して下さい as another say. But your example "moichido onegaishimasu" also works in this context.
4, 火 is fire. 火事 is used when fire is burning a house or other architectures.