So good that … / Such a success that … (and the emphatic "So great was …") (C1 Grammar)

Use so right before a describing word standing on its own — an adjective (so good) or an adverb describing an action (so quickly) — with no noun after it, then that + the result: so good that everyone agreed. It means the quality was strong enough to cause something; the that-clause names what it caused.

so + a describing word + that (the result)

• The presentation was so compelling that every investor signed up.
• She spoke so persuasively that the committee changed its decision.
• The room was so quiet that you could hear a pin drop.
• Not "It was so a good idea that everyone agreed." — say "It was such a good idea that everyone agreed.". So goes before a describing word alone (so good); once a noun follows (a good idea), switch to such.
• Try it: The evidence was ___ convincing that the jury reached a quick verdict. (so / such / such a) Answer: so — Convincing is a describing word standing on its own here, with no noun after it, so it takes so: so convincing that. "Such" and "such a" need a noun to go in front of (such convincing evidence), but here the sentence stops at the describing word.

such + a thing + that (the result)

Use such before a noun phrase (often with a or an and a describing word): such a rapid expansion that …. Then that + the result. It means the same as so … that — a quality strong enough to cause something — but the presence of a noun is what makes you choose such, not so.

• It was such a rapid expansion that the company needed more staff.
• There was such enthusiasm that the event was extended by a day.
• Wei gave such a clear answer that no one had questions.
• Not "It was so an impressive result that they celebrated." — say "It was such an impressive result that they celebrated.". A noun follows (an impressive result), so use such, not "so".
• Try it: It was ___ a memorable performance that the audience gave a standing ovation. (such / so / so much) Answer: such — A noun phrase follows (a memorable performance), so it takes such: such a memorable performance that. "So" goes before a describing word alone, not before a/an + a noun. "So much" goes before an uncountable noun (so much noise), not before a + a single thing.

so much / many / few / little + a noun + that

Use so — never such — before an amount word (much, many, few, little) plus its noun, then that + the result: so many delays that the launch slipped. The amount word carries the force and so turns it up. Pair much/little with uncountable nouns (so much noise, so little time) and many/few with countable ones (so many errors, so few staff).

• There was so much demand that the servers crashed.
• The team made so few mistakes that the audit passed instantly.
• He had so little time that he skipped the summary.
• Not "There were such many applicants that the deadline moved." — say "There were so many applicants that the deadline moved.". Before an amount word (many, much, few, little), use so, not "such" — "such many" is never used.
• Try it: With amount words like much, many, few, little, you use so. Which one fits? (There were so many applicants that the deadline was extended. / There were such many applicants that the deadline was extended. / There were so applicants that the deadline was extended.) Answer: There were so many applicants that the deadline was extended. — With amount words (much, many, few, little), use so: so many applicants that. "Such many" is not used; such goes before a describing word + noun, not before many. The third option drops the amount word, leaving so directly before the noun, which this pattern does not allow.

So great was … / Such was … — the emphatic twin

A plain so … that or such … that sentence has an emphatic twin: move the quality to the front and swap the subject and verb. Plain: The demand was so great that … → Emphatic: So great was the demand that …. Once a noun follows, front such instead: Such was the demand that …. It works best with a full noun subject like the demand; with a pronoun ("So tired was he") it sounds archaic. Reach for the fronted version in a report, a speech or an essay, where the quality should land first; in conversation the plain form is the better choice.

• So persuasive was her argument that no one objected.
• So rapid was the collapse that regulators were caught off guard.
• Such was the public outcry that the policy was withdrawn within days.
• Not "So great the demand was that they extended the deadline." — say "So great was the demand that they extended the deadline.". After fronting so + describing word, flip the subject and verb (was the demand); keeping normal order (the demand was) undoes the emphasis and is ungrammatical here.
• Try it: Rewrite for formal emphasis: 'The silence was so complete that we hardly breathed.' Which opening is correct? (So complete was the silence that we hardly breathed. / So complete the silence was that we hardly breathed. / So the silence was complete that we hardly breathed.) Answer: So complete was the silence that we hardly breathed. — Emphatic fronting moves the quality to the front (so complete) and then flips the subject and verb: so complete was the silence. The second option keeps normal order (the silence was), which cancels the inversion this pattern requires. The third splits so from its describing word — so must sit directly in front of complete.
• Try it: ___ that the whole town turned out. Build the emphatic, inverted opening: start with the strong quality, then flip the subject and verb. Answer: So great was the excitement — Emphatic fronting puts the quality first (So great), then inverts subject and verb (was the excitement): "So great was the excitement that the whole town turned out." The tile "the excitement was" keeps ordinary order and loses the inversion, and "such" would need a noun straight after it (such excitement), not a describing word like great.