¿Se dice so a good idea o such a good idea?

Such a good idea. So nunca va directo antes de a o an.

¿Such va con sustantivo plural?

Sí, sin artículo: such good ideas, such nice people.

Gramática comparada

So va antes de un adjetivo o adverbio solo. Such va antes de un sustantivo, con o sin adjetivo.

So / Such
So	Such
The film was so boring. (adjetivo solo)	It was such a boring film. (adjetivo más sustantivo)
She speaks so fast.	She's such a fast speaker.
It was so expensive that we didn't buy it.	It was such an expensive bag that we didn't buy it.

So y such comparados
Punto	So	Such
Va antes de	Un adjetivo o adverbio solo	Un sustantivo, con o sin adjetivo
Patrón	so + adjetivo o adverbio	such (a/an) + (adjetivo) + sustantivo
Ejemplo	The food was so good.	It was such good food.
Con sustantivo singular	No se usa directamente	such a/an + sustantivo: such a good idea

Tan cubre los dos usos en español, así que el inglés obliga a elegir: tan rico es so good antes de un adjetivo solo, pero such a good meal antes de un sustantivo.
No digas so a good idea. Decí such a good idea, o the idea was so good.
Such va antes del artículo: such a day, no a such day.
So...that y such...that introducen un resultado: it was so hot that we stayed in, it was such a hot day that we stayed in.

¿Se dice so a good idea o such a good idea?: Such a good idea. So nunca va directo antes de a o an.
¿Such va con sustantivo plural?: Sí, sin artículo: such good ideas, such nice people.

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