1. This book is really fun.

2. This book is real fun.
3. This book is great fun.

(Which one is correct? Are all corect? What is the part of speech of 'fun'?)

4. Shine a light on the back of the stage.

5. Shine a light at the back of the stage.

6. Shine a light in the back of the stage.

(Are all the same and grammatical? Do they have differences?)

1 and 3 are fine; 2 is not grammatically correct.

"Fun" is a noun.
http://www.answers.com/topic/fun

4, 5, 6 -- I've answered these before.

1. This uses the adverb "really"

2. This uses the adjective "real" and is not good.
3. OK and fun is a noun

4. the light has to be above

5. the light is somewhere toward the back

6. the light could be anywhere

4, 5, 6 are all fine grammatically but there "could" be differences.

Sra

1. "This book is really fun."

2. "This book is real fun."
3. "This book is great fun."

All three sentences are grammatically correct, but they have slightly different meanings.

In sentence 1, "really" is used as an adverb to intensify the adjective "fun," indicating that the book is very enjoyable.

In sentence 2, "real" is a colloquial variation of "really," often used in informal speech. So, "real fun" is similar to "very fun" or "really fun."

In sentence 3, "great" is an adjective that intensifies the noun "fun." It suggests that the book provides a greater degree of enjoyment compared to just being "fun."

The part of speech of 'fun' in these sentences is a noun. In each sentence, 'fun' is the object of the linking verb 'is.'

Now, let's move on to the next set of sentences:

4. "Shine a light on the back of the stage."
5. "Shine a light at the back of the stage."
6. "Shine a light in the back of the stage."

These sentences have slight differences in meaning and usage.

In sentence 4, "on" is used to indicate that the light should be directed toward the back of the stage as a whole.

In sentence 5, "at" is used to indicate that the light should be focused specifically on the area or location at the back of the stage.

In sentence 6, "in" suggests that the light should be positioned within the back area of the stage.

So, while all three sentences are grammatically correct, they convey slightly different instructions or emphasis on where the light is directed. The choice of preposition depends on the desired meaning and specific context.