Such vs so

SUCH
Such is used with nouns and noun phrases. It comes before a\an:
Such people
Such interesting ideas
Such a decision (not a such decision)

SUCH
Such can mean “like this\that”, “of this\that kind”. This is most common in a formal style, with abstract nouns:
The committee wishes to raise fees. I would oppose such a decision.
There are various forms of secret writing. Such systems are called ‘codes’.
In an informal style, and with concrete nouns, we prefer like this\that or this\that kind of:
…systems like this are called…
He’s got an old Rolls-Royce. I’d like a car like that. (not I’d like such a car.)

SUCH
Such is used to talk about a high degree of some quality. In this sense, such is common before adjective + noun.
I’m sorry you had such a bad journey.
It was a pleasure to meet such interesting people.
Such is also possible with this meaning before a noun alone, when the noun has an emphatic descriptive meaning:
I’m glad your concert was such a success.
Why did she make such a fuss about the dates?

SUCH VS VERY, GREAT ETC.
Very, great and similar words are also used to talk about a high degree of some quality. The difference is that they give new information; such (like this\that) normally refers to information that is already known:
I’ve had a very bad day. (giving information)
Why did you have such a bad day? (the information is already known)
The weather was very cold. (giving information)
I wasn’t expecting such cold weather. (the information is already known)
There was great confusion. (giving information)
Why was there such confusion? (the information is already known)
However, in a very informal style such also can be used to mean “very” or “great”, especially, in exclamations:
She has such a marvelous voice! He’s such an idiot!

SUCH VS VERY, GREAT, ETC.
Structures with very cannot be followed directly by that-clauses. Instead, we can use such….that.
It was such a cold afternoon that we stopped playing. (not It was a very cold afternoon that…)
There is also a structure with such followed by …. + as to + infinitive.
This is formal and not very common:
It was such a loud noise as to wake everybody in the house.
Less formal –(… such a loud voice that it woke….)

SUCH AS
Such as is used to introduce examples:
My doctor told me to avoid fatty foods such as bacon or hamburgers.

SUCH – AND- SUCH
There is an informal expression: such-and-such
She’s always telling you that she’s met such-and0such a famous person. (=one or other famous person)

SUCH BEFORE (ADJECTIVE+) NOUN
We use such before a noun (with or without an adjective):
They’re such fools. (NOT: They’re so fools.)
It was such good milk that we couldn’t stop drinking it. (NOT: It was so good milk that…)
Such comes before a\an:
She ‘s such a baby.
I’ve never met such a nice person. (NOT….a such\so nice person).

SO BEFORE ADJECTIVES, ADVERBS, ETC.
We use so before an adjective alone (without a noun) or an adverb:
She’s so babyish. (NOT She’s such babyish.)
The milk was so good that we couldn’t stop drinking it.
Why do you talk so slowly?
We can also use so before much, many, few and little:
We’ve got so much to do, and so little time.
We use so much, not so, before comparatives:
I’m glad you’re feeling so much better. (NOT…so better).