Future simple: be going to and Will

to express a situation or intention in the future

to make a prediction

to express willingness / unwillingness to do something

to make promises or threats

when making a spontaneous decision

When we have already decided or we INTEND to do something in the future. ( Prior Plan) The decision has been made before the moment of speaking.

When there are definite signs that something is going to happen. ( Evidence)

When something is about to happen: Get back!