Separable or inseparable phrasal verb

A further way of considering phrasal verbs is whether they are separable or inseparable. In inseparable verbs, the object comes after the particle:
"The gas gave off fumes."
"On Fridays, we look after our grandchildren."

Separable verbs have several ways separating verb, particle and object. Usually, the object comes between verb and particle:
"They let the man through at the barrier."

However, with some separable verbs, the object can come before or after the particle:
"Switch the light off."
"Switch off the light."
"Switch it off."