Saludos.
Los apostrofes son utilizados para hacer contracciones.
En los ejemplos que mencionas haces referencia a los siguientes : - isn't = is not - we're = we are - doesn't = does not - that's = that is - wasn't = was not - aren't = are not
Ahora, algunos ejemplos podrían ser : - She isn't here.
(Ella no está aquí) - We're studying maths.
(Estamos estudiando matemáticas) - He doesn't like this place.
(A él no le gusta este lugar) - That's what I want.
(Eso es lo que quiero) - I wasn't playing videogames.
(No estaba jugando videojuegos) - They aren't good enough for this.
(No son lo suficientemente buenos para esto).