To be honest, you need to forget that these words are synonyms for a moment, and look up each word in a dictionary (or two). Yes, this takes time, but it will solve your problem permanently. You will understand the differences by looking at the differences.

Here's another interpretation of them: - affect: CAUSE A CERTAIN CHANGE to, making the thing/person changed. - effect: BRING ABOUT an outcome, condition, result, etc. - influence: INTERFERE WITH and CHANGE, slowly or as a side effect. - impact: CAUSE A SUDDEN, BIG CHANGE on a thing or person. Examples: - This policy will affect a great majority of the population. - This policy will effect a welcome change for the nation. - This policy will influence the foreign policy of surrounding nations. - This policy will impact the economic recovery.

So firstly, 'affect' is a verb: 'to affect' and is different from 'effect', a noun, so when we see 'effect' it is the 'impact' on something. Let's use an example: "What are the effects of drink driving? Driving concentration has been affected as well as reaction time" I just used the third word in your list which is used for two meanings : like above for effect and also a physical impact, when an aeroplane( airplane in US spelling) hits the ground it makes an impact. Finally, influence is a verb and a noun, we use this term in a more metaphorical (from metaphor implying something different, non-literal) or cultural sense. An example of this is "people in the Renaissance were influenced by ideas of the ancient Greek and Roman thinkers and artists".

Thanks for all your help.. :)