We usually use "so" before adjectives that don't need an article, and "such" for those that use the article "a/an." Examples: The diamond is so valuable that it needs to be kept in a safe. It is such a valuable diamond that it needs to be kept in a safe.

So is used mostly with adjectives and adverbs. Such is used mostly with nouns.

'So' is used with adjectives to show extremes. 'This bag is so heavy.' Sometimes we combine this with 'that' to show extremes which lead to certain results. 'This bag is so heavy that I'm going to drop it soon!' 'So' is also used with adverbs to show extreme actions. 'He paints so well!' And we can also combine this with 'that' to show extreme actions which have certain results. 'He paints so well that they offered him a scholarship.' So can be combined with much, many, few or little to show extremes of quantity.'He has so many brothers!' This can also be combined with that..'He has so many brothers that they have to sleep five in a room!' 'So' can also be used with adverbs of frequency to say how often someone does something. 'They go out so often!' 'Such' can be combined with an adjective and noun to show extremes. 'They have such a big house!' This can also be combined with 'that' 'They have such a big house that she has to have two cleaners!' 'Such' can be used with a judgemental noun to give emphasis. 'He is such an idiot!' This can also be combined with 'that'. 'He is such an idiot that nobody would give him a job!' In all the above we usually end with an exclamation mark (!) 'Such' can also mean 'this type of' or 'that type of' 'He had never made such mistakes before.'

so + adjective such + noun