简单来说，希腊原文圣经里，有定冠词和没定冠词的 God 都有用来指 神。Joh 1:1里用了有定冠词的 God，也用了没定冠词的 God，Dr. John Ankerberg, Dr. John Weldon说可能是为了区别圣子与三位一体 （http://www.johnankerberg.com/Articles/bible-for-dummies/BD0805W3.htm）。
Dr. John Bechtle 说得更易懂：http://www.christiananswers.net/q-acb/acb-r001.html

• John 1:6 para theou—no definite article
Joh 1:6 有一个人，是从神那里差来的，名叫约翰。

• John 1:12 tekna theou—no definite article
Joh 1:12 凡接待他的，就是信他名的人，他就赐他们权柄，作神的儿女。

• John 1:13 ek theou—no definite article
Joh 1:13 这等人不是从血气生的，不是从情欲生的，也不是从人意生的，乃是从神生的。

• John 1:18 Theon—no definite article
Joh 1:18 从来没有人看见神，只有在父怀里的独生子将他表明出来。

• John 1:23 odon Kuriou—no definite article
Joh 1:23 他说：「我就是那在旷野有人声喊著说：『修直主的道路』，正如先知以赛亚所说的。」

http://www.christiananswers.net/q-acb/acb-r001.html
Should John 1:1 be translated,“The Word was God” or “The Word was a god”?

Jehovah's Witnesses deny the deity of Christ, and claim that John 1:1 merely calls him “a god,” but not full deity. They rest their case on three facts of Greek grammar:

There is no such word as "a" or "an" in Greek, so we sometimes have to add "a" to translate into English, (Acts 28:6).
The Greek word used here (theos) has two meanings: usually the supreme God revealed in ????ure, but sometimes lesser beings like the gods of Greek mythology.
The Greek word "the" is often attached to the word "God" or theos, but it does not appear in John 1:1. Hiding behind the Witness rendering of the verse is an unspoken equation: God + "the" (ho theos) = Jehovah, the Almighty God, God - "the" (theos) = a created being with divine qualities. Witnesses claim that the apostle John deliberately omitted a "the" in the final phrase to show the difference between God and the Word. As the New World Translation (p. 775) explains:

John's inspired writings and those of his fellow disciples show what the true idea is, namely, the Word or Logos is not God or the God, but is the Son of God, and hence is a god. That is why, at John 1:1,2, the apostle refers to God as the God and to the Word or Logos as a god, to show the difference between the Two.
Is this the proper translation?

No. The equation underlying the Witness rendering breaks down within a few verses. John 1:18 contains theos twice, without “the” either time. According to Watchtower assumptions, we would expect to translate both as “god” or “a god.” Instead, the New World Translation says "God" the first time and "god" the second time. The context overrules their rule.

Why did John choose not to put “the” on the word “God”?

To show which word was the subject of the sentence. In English, we can recognize the subject of a sentence by looking at word order. In Greek, we must look at the word endings. John 1:1 is trickier than most verses, because both “God” (theos) and “Word” (logos) have the same ending. The usual way to mark off the subject clearly was to add “the” to the subject and leave it off the direct object. That is precisely what John did here.
To conform to standard Greek grammar. E.C. Colwell demonstrated in an article in the Journal of Biblical Literature in 1933 that it was normal practice to omit "the" in this type of sentence. John was simply using good grammar, and making it clear that he intended to say, “The Word was God” rather than “God was the Word,” a statement with some theological drawbacks. John constructed his sentence in the one way that would preserve proper grammar and sound doctrine, declaring that “the Word was God.”
Author: Dr. John Bechtle

http://www.johnankerberg.com/Articles/bible-for-dummies/BD0805W3.htm

In John 1:1, the New World Translation of the Jehovah’s Witnesses inserts the word "a" in an attempt to deny Christ’s deity: "In (the) beginning the Word was, and the Word was with God, and the Word was a god." (New World Translation [NWT])

The same verse in the New American Standard Version reads this way: "In the beginning was the Word, and the Word was with God, and the Word was God."

The transliterated Greek of this verse looks like this:

En arche en ho logos kai ho logos
In beginning was the Word and the word

en pros ton theon kai theos en ho logos
was toward the God and God was the Word

In essence, the Watchtower Society claims it can translate theos as "a god" because there is no definite article before this usage of theos (God) in the last clause of John 1:1. Note that the first use of the term God (pros ton theon) has the article (ton—the). The second use simply states kai theos ("and God," not "and the God"). Because it does not say "and the God" Jehovah’s Witnesses argue they are free to interpret this second usage of God as figuratively meaning a lesser deity, "a god"—signifying Christ’s exalted status, even though he is still only a creature. Their main concern here is to escape the clear meaning of this passage. Christ is here called theos, God.

The difficulty is that, had the apostle John used the article, he would have declared that "the God was the Word." Had he done so, he would have confused the persons of the Trinity and supported modalism (in the early church known as the heresy of Sabellianism1). In other words, to declare that "the God was the word [Jesus]" would have stated that all of God—i.e., the whole trinity—was Jesus. This would have supported modalistic belief that there is only one Person in the Godhead (i.e., Jesus) and that the terms Father, Son and Spirit in ????ure only refer to modes or offices of the one God who exists as one person.

The apostle John had to make a finer distinction and, on the one hand, clearly declare that the person of Jesus was deity, but, on the other, not make it seem as if all three persons in the Godhead were to be considered the same as the person of Jesus. To make this fine distinction he had to use the exact wording he used.

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（余下的文章请看连接）

Nevertheless, even if we were to assume the truth of what the Watchtower Society claims in their appendix, they have violated their own "rule" in John 1:1 94% of the time. Robert H. Countess, writing in The Jehovah’s Witnesses’ New Testament, documents this in detail.9 In John 1 alone they violate their principle at least five times. Checking their interlinear (pp. 417-419) we see the following:

• John 1:6 para theou—no definite article

• John 1:12 tekna theou—no definite article

• John 1:13 ek theou—no definite article

• John 1:18 Theon—no definite article

• John 1:23 odon Kuriou—no definite article

If the absence of the article demands the "a god" rendering, why is it not so rendered here? In fact, where is it in 94% of the instances of such construction in the NWT? Clearly translating John 1:1 "a god" is not only a violation of Greek grammar, it is unjustified even in light of the vast majority of their own translation. Obviously then, in John 1:1 (NWT), the translation should be "God," not "a god."

(As an aside, the NWT at John 1:23 translates the Greek kurios (Lord) as "Jehovah," since it is a clear reference to Jehovah God from Isaiah. Yet, according to their John 1:1 rendering, with no definite article it should be "a Jehovah." If "a god" must be different from God, "a Jehovah" must then be different from Jehovah. At this point we would have three Gods: "Jehovah," "a god" and "a Jehovah.")