In the Our Father, Jesus taught his disciples to pray for deliverance from the "Evil One," not merely from "evil" as such. Why hasn't this prayer been translated accurately? Shouldn't we be praying what Jesus taught us to pray, word for word?
First, Jesus probably didn’t teach the Our Father in Greek (the language we have the Gospels in) but in Aramaic, so any English version is a translation of a translation.
Second, you can’t always translate word for word because the results would be awkward or even unintelligible. Translation invariably involves splitting words in two, combining them, dropping them out, adding them, or rearranging them, depending on the rules of the language you are translating into. To refuse to do any of these things would result in a lousy translation.
If it were translated in a strictly word-for-word manner, the Our Father would read like this:
Father of-us the in the heavens, let-be-made-holy the name of-you, let-come the kingdom of-you, let-come-about the will of-you, as in heaven also upon earth. The bread of-us the daily give to-us today and dismiss to-us the debts of-us, as also we dismissed to-the debtors of-us. And not into-bring us into trial, but deliver us from the evil.
Third, languages obey different rules and translations cannot be done in a strictly word-for-word manner. Translators are sometimes confronted with situations where they have to make a decision about how to render something, because the original can have more than one meaning, and there is no good way to express this ambiguity in the translation.
An example of this is the one you mention. Greek tends to use the definite article (in English, the word the) much more than we do. For example, you might read about "the Paul" going and saying something to "the Peter" about "the Jesus the Christ." Because it gets used in Greek so frequently and in ways that English doesn’t use it, translators have to decide when to drop it and when to include it in their translations.
One such case is at the end of the Our Father. The Greek literally says "the evil." Does this mean "the evil one" or does it just mean "evil"? Because of the way the article is used in Greek, it can have either meaning. In fact, because language can be deliberately ambiguous, it could mean both—i.e., "deliver us from the evil one and from evil in general."
The English translators of the Our Father chose one option rather than another, but it isn’t an option where you can look at the Greek and say, "That’s wrong. That’s not what the Greek says." On the other hand, one can look at the "forgive us our trespasses as we forgive those who trespass against us" and say "That is wrong. The words in Greek are clearly debts and debtors, not trespasses and those who trespass against." This criticism affects only the English version of the Our Father. (The Pater Noster, for example, has it right: debita and debitoribus.)