So and such are both used to introduce result clauses with 'that,' but they follow different grammatical patterns. Understanding when to use each is essential for advanced formal writing.
So + adjective/adverb + that
Use so directly before an adjective or adverb (without a following noun) to introduce a result clause.
The presentation was so compelling that every investor signed up.
She spoke so persuasively that the committee changed its decision.
Such + (a/an) + noun phrase + that
Use such before a noun phrase (optionally with adjectives and articles) to introduce a result clause.
It was such a rapid expansion that the company needed more staff.
There was such enthusiasm that the event was extended by a day.
So much/many/few/little + noun + that
Use so with quantifiers (much, many, few, little) before nouns.
There were so many applicants that the deadline was extended.
She had so little time that she could not review all the documents.
Common mistakes
Not "It was so a good idea that everyone agreed." — say "It was such a good idea that everyone agreed.". Before 'a/an + adjective + noun,' use 'such,' not 'so.' Use 'so' only before adjectives/adverbs without a following noun..
Practise it
Quick test: Is there a noun right after? Use 'such.' Is it just an adjective or adverb? Use 'so.' Is there a quantifier (much/many/few/little)? Use 'so.'