Such is a determiner; so is an adverb. They often have the same meaning of ‘very’, 'really' or ‘to this degree’:
Those are such good ideas.
Those ideas are so good.
We use such + noun phrase and so + adjective or adverb phrase:
✅ This is such a great opportunity for our team.
❌ Not: This is so great opportunity.
✅ Step 4 was so challenging but we managed to find a viable solution. (so + adjective)
❌ Not: Step 4 was such challenging.
✅ How did you manage to complete this project so quickly? (so + adverb)
❌ Not: How did you manage to complete is such quickly?
We use such, not so, before a noun, even if there is an adjective before the noun:
✅ Thank you very much for accommodating our needs at such a short notice.
❌ Not: ...at so short notice.
✅ After go through references, we managed to find such a straightforward method that the whole timeline could potentilly be pushed forward by 2 weeks.
❌ Not: ...so straightforward method.
We use such, not so, before a noun phrase with the indefinite article a/an - this is often a "noun" that shows our opinion:
✅ This project is such a success!
❌ Not: This project is so success …
We use so, not such, before adjectives:
✅ Thank you. You’re so kind.
❌ Not: You’re such kind.
✅ It's so great to hear that you have decided to go with our team!
❌ Not: It's such great to hear.
We use so, not such, before adverbs:
✅ She always dresses so elegantly.
❌ Not: She always dresses such elegantly.
We can use 'so... that...' and 'such... that...' to show that there is a certain result. (We can't use 'very' or 'really' in this way.)
✅ The timeline was so tight that we had to work overtime.
✅ It was such a complicated issue that we had to invite an expert in the field, but we did manage to resolve it.
We use 'so' before 'much / many / little / few' with and without a noun. This shows that the amount is really a lot or really little. As usual, we use a plural noun after 'many' and 'few' and an uncountable noun after 'much' and 'little'.
✅ The yield was so low. We had to restart.
✅ There were so few issues. We managed to finish it ahead of schedule.