First, Firstly

[Emanuelli]

Hello again:

1. Please, let me know if first/firstly, second/secondly are adverbs. I mean, may I use any when I make sentences? Example:
* Firstly, let me tell you about my holiday.
* First, let me tell you about my holiday.

2. If I want to say the number of the room in a school, can I say: "Class 01", "Class 02", or should I say: "classroom 01", "classroom 02"...

3. I know that when I use year/week, I say next Sunday, next week, next year. But I have already seen in the next century. Can I say: next century?
When do I use the preposition?

Thanks.
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[RonBee]

Hello again:

1. Please, let me know if first/firstly, second/secondly are adverbs. I mean, may I use any when I make sentences? Example:
* Firstly, let me tell you about my holiday.
* First, let me tell you about my holiday.

2. If I want to say the number of the room in a school, can I say: "Class 01", "Class 02", or should I say: "classroom 01", "classroom 02"...

3. I know that when I use year/week, I say next Sunday, next week, next year. But I have already seen in the next century. Can I say: next century?
When do I use the preposition?

1. I suppose you could say they are adverbs, but why use them at all? (First, second, third are better style.)

2. Use classroom.

3. Yes, you can say next century.

What preposition?