what is the difference in meaning between these 2 sentences:
We saw him skate
We saw him skating
An ing-form always stresses the activity. It doesn't chance the meaning of the sentence, it just ephasises the action. Although, it doesn't change the meaning, the use of the infinitive or the ing-form does have an impact on the receiver of your message.
You use the infinitive when you want to share some regular information.
You use the ing-form when you want the stress the activity of the person. eg: Why wasn't he in class the other day ? Then you can reply with "We saw him skating". In this case it far more the better option than using the infinitive. Most of the time the reason for using the ing-form is linked to something said/asked before.