Subtract- To remove something from something else (usually abstract things, like numbers) Mitigate- To lessen the damage that something does Alleviate- To rid oneself of a problem Reduce- To proportionally decrease the total quantity of something "Reduce" and "subtract" are very similar words; the big difference between them is that to subtract something would be to literally remove one part of a whole, while to reduce something would be to "shrink the size" of the whole. All four are used in some common fashion, with "mitigate" being the rarest word. You'd use "alleviate" when talking about body aches or illnesses (e.g. "This pill will alleviate your headache."). "Subtract" would most commonly be used when talking about literal numbers. "Reduce" is likely the most common and varied of the four, and while I gave you one major definition, it can vary depending on the situation, so you'll just have to learn that through immersion. "Mitigate" is something you'd hear in business meetings or news reports (e.g. "Firefighters are mitigating the damage from the latest flash fire in Colorado," or, "We need to mitigate our quarterly losses as much as possible."), so you don't have to really worry about using it yourself, but it's good to know what it means.