due to / because of
After a linking verb vs. before a reason.
Due to means caused by and follows a linking verb such as was: The delay was due to rain. Because of explains why an action happened: They left because of the rain. A common error is to use due to in place of because of, as in They canceled due to rain. Due to must describe a noun that comes before it. In The delay was due to rain, the phrase describes the delay. A verb needs because of instead: They canceled because of rain.
Examples
UseThe cancellation was due to rain. (caused by rain)
UseThey called off the game because of rain. (why they called it off)
AvoidThey called off the game due to rain. (due to cannot explain an action)
Dictionaries now accept due to in place of because of. However, many people still consider this use an error, and they judge the speaker who uses due to this way.