Distinguish the mens rea element of a crime from the actus reus element.

The mens rea requirement of a crime is the mental element part. For example, having the intention or knowledge of the wrongdoing. Differently, the actus reus element is the conduct part of the crime, for example, the action itself or the omission to act. In order to establish liability, it is always important to distinguish the actus reus and mens rea elements of an offence. This is important because generally, with the exception of strict liability crimes, you will not be liable if you act without mental fault. 

To explain these concepts, it will be helpful to use an example. I will use the offence of murder as my example here. The actus reus element for murder is the the 'killing' of the victim: the act that results in the death of an individual. The mens rea element is: the intent to kill or cause grievous bodily harm to that person. If I am ignorant that my act in throwing a stone in someone's direction, which then hits them on the head, will result in their death, I will not be liable for murder (though I may be for manslaughter) because the mens rea element required is missing.