This is not what I ordered.

Kore wa watashi ga chuumonshita mono dewa arimasen. : This is not what I ordered.
kore : this
watashi : I
chuumon : order(noun)
suru : do
(shi-nai,shi-masu,suru,suru-toki,sure-ba,shi-ro,shiyou,shita)
chuumonsuru : order(verb)
mono : thing
arimasu : is / are
arimasen : is not / are not
...dewa arimasen : is not..../ are not....

Watashi ga chuumonshita : I ordered.
watashi ga chuumonshita mono : what I ordered, a thing which I ordered
Kore wa watashi ga chuumonshita mono dewa arimasen. : this,I,ordered,thing,not : This is not what I ordered.