"This is extremely useful."
Translation:C'est extrêmement utile.
In a toy store, a little girl and her mother :
Mum I want this game, this doll and these balls. (all of them are in front of her)
Maman je veux ce jeu, cette poupée et ces ballons. (jeu:masc.sing., poupée:fem.sing., ballons:masc.plur.)
Mum, showing a toy next to her : I prefer this. Je préfère ceci.
and showing another one a bit further : or maybe that. ou peut-être cela.
We often use ça instead of ceci and cela, but I assume DL won't appreciate.
there are also celui-ci, celle-ci, celui-là, ceux-là but it is another story.....