This is a pen.

pen; pen
kore; this
are; that
sore; it
korera; these
arera; those
desu; be(is , are)

Kore-wa pen desu.; This is a pen.
Are-wa pen desu; That is a pen.
Sore-wa pen desu.; It is a pen.

Korera-wa pen desu.; These are pens.
Arera-wa pen desu.; Those are prens.

No plural in Japanese noun.
Sometimes "-ra" which is added just behind a noun means plural.

shounen; boy

Kare-wa shounen desu.; He is a boy.
Karera-wa shounen desu.; They are boys.