1. This book is really fun.

Question

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

(I have a question. You said #1 and #2 are right, but that #2 is not right. What about 'good' fun? Is it right to use 'good fun' instead of 'great fun'? If #3 is right, why is #2 not right? In Sentence 3, 'fun' is used as a noun, for 'great' modifies 'fun'. However in Sentence 1, fun is used as an adjective, because 'really' is an adverb and modifies 'fun.' Right? What about Sentence 2. In Sentence 2, can't 'real' modify 'fun'? If 'real' can modify 'fun', fun can be used as a noun. Right? I anticipate your answering the questions. Thank you for your help.)

Answer 1

From the dictionary, here is the word fun:

fun
   /fʌn/ Show Spelled [fuhn] Show IPA noun, verb,funned, fun·ning, adjective
–noun
1.
something that provides mirth or amusement: A picnic would be fun.
2.
enjoyment or playfulness: She's full of fun.
–verb (used without object), verb (used with object)
3.
Informal. joke; kid.
–adjective
4.
Informal. of or pertaining to fun, esp. to social fun: a fun thing to do; really a fun person.
5.
Informal. whimsical; flamboyant: The fashions this year are definitely on the fun side.

So, if you are using fun as a noun in #2, yes, that would be correct. When I first looked at it, it didn't look as good as #1. However, you ARE right in that case.

Sra