'So' intensifies adjectives or adverbs. 'Such' intensifies noun phrases, often with an adjective inside. Both words express strong degree, but they attach to different structures in the sentence. Choosing correctly depends on what part of speech follows.

Rule
Use 'so' before an adjective or adverb.
Use 'such' before a noun phrase, with or without an adjective.

'So' is used directly before an adjective to increase its strength. It can also appear before an adverb to intensify the manner or degree of an action. 'So' often introduces a result clause with 'that,' but the basic rule is about what it modifies. The word after 'so' is typically an adjective or an adverb, not a noun.

Word/Phrase	Definition
so + adjective	'So' intensifies an adjective.
so + adverb	'So' intensifies an adverb.

'Such' is used before a noun phrase to express strong quality or degree. The noun phrase may include an adjective, creating patterns like 'such a good idea' or 'such interesting people.' The key is that a noun is present, possibly with describers. 'Such' can also lead to a result with 'that,' but its main job is to intensify the noun phrase.

Word/Phrase	Definition
such + noun	'Such' intensifies a noun phrase.
such + adjective + noun	'Such' intensifies a noun phrase with an adjective.
such + plural noun (no a/an)	'Such' intensifies a plural noun phrase.

When 'such' modifies a singular countable noun, it normally uses 'a' or 'an' after 'such.' The structure is 'such a' or 'such an' plus an optional adjective and a singular noun. Plural nouns and uncountable nouns do not take 'a' or 'an' in this pattern. The choice between 'a' and 'an' follows the usual rules of English articles.

Word/Phrase	Definition
such a + singular noun	Use with a singular countable noun.
such a + adjective + singular noun	Use with an adjective and a singular noun.
such + uncountable noun	Use without 'a' for uncountable nouns.
such + adjective + plural noun	Use without 'a' for plural nouns.

'So...that' links an adjective or adverb to a result, forming a cause-and-effect meaning. 'Such...that' links a noun phrase to a result, keeping the noun as the focus. Both structures explain why something happens, but the choice depends on whether the degree is attached to an adjective/adverb or to a noun phrase. The word immediately after 'so' or 'such' shows which pattern you are using.

Word/Phrase	Definition
so + adjective + that	The adjective is intensified and leads to a result.
such + (a) + adjective + noun + that	The noun phrase is intensified and leads to a result.

Use 'so' to intensify adjectives and adverbs, and use 'such' to intensify noun phrases. 'Such' often appears with 'a' or 'an' before a singular countable noun. In result clauses, 'so...that' attaches to an adjective and 'such...that' attaches to a noun phrase. Accurate choice relies on recognizing whether you are emphasizing a describing word or a noun.