The confusion of words is normally very easily corrected. A misused word usually has a very clear difference in meaning from the correct word:
>their (possession) - there (location) - they're (state of being)
>bare (expose[d]) - bear (animal, "smarter than the average")
Most cases are similar to the examples above, but one pair has given me trouble in the past. I wasn't quite sure why, but now I think I've gotten a handle on it. I'm talking, of course, about affect vs effect. Tons and tons and tons of literature exists on the topic (all saying the same thing). The knowledge is certainly obtainable if you really want to understand the difference between the two. However, the very fact that there are VOLUMES of explanations published on these words suggests that there is some kind of lexical anomaly surrounding them.
The reason is actually quite simple. Imagine, if you will, that words are people. Different personalities, different beliefs, different appearances BUT they can sometimes have similar qualities. You know the difference between two of your friends but occasionally you will call one friend by another friend's name because they have a similar hair style. A brain fart. Not really a big deal- just pay for their coffee and all will immediately be forgiven. I would wager that the same psychology is behind confusing to and too.
Now imagine that two words are adult identical twins. I emphasis adult because I want you to picture identical twins that have developed unique personalities and have acquired minor physical differences. If you just met them or just saw a picture of the two of them, you would have no idea who was who. However, if you were consistently exposed to them and took the time to get to know them, then you would clearly see two completely different people. I have two brothers-in-law, Josh and Caleb, who are identical twins. When I first met them I had to laboriously (and awkwardly) use context clues to figure out which one was which; now I can tell whom I'm looking at just by glancing at the back of their head from 50 yards away. Such is the psychology of affect and effect.
Josh on top, Caleb on bottom
The similar appearance of affect and effect is obvious. They are minimal pairs differing only in their initial vowel. The meanings of these words are also pretty similar. Yes, they have different functions. Effect is almost always a noun. Affect is almost always a verb. However, a simple shift in the focus of a sentence dictates which word we use. Affect makes the change, effect IS the change. Take the above image into consideration. We can write two sentences:
Josh affected Caleb's posture.
There was a negative effect on Caleb's posture.
These sentences are talking about the same subject matter but have distinctly different focuses. We need to familiarize ourselves with these words the same way we would with twins in order to use and identify them properly.
Now, for you super English nerds, I realize this simile can be misleading and is incomplete. Yes I know affect and effect have different entomologies (though both are French- the words AND Josh and Caleb). The words aren't 'blood relatives' like identical twins are. I'm simply making an analogy about the contemporary appearance and understanding of the words and adult identical twins.
Also, I realize there are some other uses for affect and effect that were not discussed here. For those interested in owning this knowledge, you can go here or here or here (among other places).