"A spoonful of sugar is enough."

Translation: Ένα κουτάλι ζάχαρη είναι αρκετό.

[J2greek4u]

Why isn't sugar declined in the genitive?

[troll1995]

When you say a cup of tea, a plate of salad, a spoon of sugar, you have to use the accusative in Greek, because it doesn't show possession. Accusative is used because a με is implied κούπα (με) τσάι, πιάτο (με) σαλάτα, κουτάλι (με) ζάχαρη etc. Genitive would mean that the κούπα τσαγιού is a tea cup, a πιάτο σαλάτας is a salad plate (meant only for salads) etc. The με thing becomes obvious when using the definite article; there you have to say θέλω το ποτήρι με το τσάι= I want the cup of tea, θέλω το πιάτο με την σαλάτα=I want the plate of salad etc. In everyday life, though, you just say Θέλω το τσάι, θέλω την σαλάτα etc

[Frances900805]

What about μια κουταλιά ζάχαρη ειναι αρκετά????

[G.Georgopoulos]

It would be "αρκετή". "Αρκετά" can be either a neuter plural adjective or an adverb.

[Karly902254]

Can you use φτάνει instead of αρκετό?

[D_..]

You can say Ένα κουτάλι ζάχαρη φτάνει / αρκεί (A spoonful of sugar suffices) not Ένα κουτάλι ζάχαρη είναι φτάνει. You can't have two verbs. Also, if you use φτάνει (or αρκεί) it's not really a translation of the original sentence.