"Airplanes are fast."

Translation: Tha plèanaichean luath.

[pi5zZA0z]

I recall that with other nouns, when you pluralize them you add an "a" at the end of the adjective. Not here though?

[silmeth]

Only the attributive adjectives – ones that directly complement nouns to form a single noun phrase – change. When an adjective complements a verb (it is the predicate of the clause) it always stays in the base form in Gaelic.

plèanaichean luatha fast planes

but:

tha plèanaichean luath planes are fast

[pi5zZA0z]

Aha, Moran taing!