So/Such

So is used before an adjective or an adverb :

• So big
• So carefully

Such followed by 'a' or 'an', is used before an adjective + a singular noun : 

• Such a long time
• Such an incredible story

Such is used alone (without a, an) before plural nouns, mass nouns and abstract nouns (information, advice, furniture, courage, generosity, interest, kindness ... )

Compare the following sentences:

• This book is so interesting.
• This is such an interesting book.
• Such interesting books are difficult to find.

• I was amazed that the crowd was so big.
• I was amazed that there was such a big crowd.
• The story was so complicated.
• It was such a complicated story.

• You are so intelligent.
• You are such an intelligent person.
• A person of such intelligence is a pleasure to meet.