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I think the words such as 'form', 'type' and 'sort' tend to negate the need to have an article before the noun that follows it.

a form of human being
a type of pen
a sort of animal

The nouns that followed the phrases 'a form', ' a type', and 'a sort' are believed to be countable nouns and due to the existence of those phrases, the need to have articles have been null (?) or has been removed.

Now, look at these please.

I believe I used this phrase to ask a question before.

... the particular form of service

Here, the word 'service' is countable I think and since we have the phrase 'the particular form', the need to place an article before the word 'service' has been removed.

But that can't be said for this I think.

... in the form of spirit

The word 'spirit' seems countable, yet the existence of the phrase 'in the form' has not removed the need to place an article before the word 'spirit' (I think). Why??
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... the particular form of service

Here, the word 'service' is countable I think and since we have the phrase 'the particular form', the need to place an article before the word 'service' has been removed.

I don't read it as countable. this particular form of service will cost you an extra $50 per hour. If it were countable, we'd say, "This particular service will cost you," or "Services like this one will cost you."

But that can't be said for this I think.

... in the form of spirit

The word 'spirit' seems countable, yet the existence of the phrase 'in the form' has not removed the need to place an article before the word 'spirit' (I think). Why??

Again, I disagree with your assumption. I think your example is uncountable. (I don't often hear this. I think the adjective is more common, not referring to an individual (countable) spirit, "in the spirit form." Here, "form" is countable - i.e., various forms are available)

If we want a countable spirit, we say, "in the form of a spirit." Hmmm. I guess that's what you're saying! Okay, maybe I get it now. This is a different usage of the word "form." We can say, "I think this guy is some type of [a] spirit." "I think this guy is some form of spirit." This follows your rule.

But when you say, "I think this guy is in the form of a spirit," it's a different ballgame. We're saying which form he is in, not what barrel his drink was drawn from.

I hope someone can say this with more clarity - or disagree with me.

- A.

BTW, the angel is the result of my putting "a" in straight brackets, meaning it was omitted. So that's the code for emoticon angel! I'm leaving it because we're talking about spirits.
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Thank you, Avangi. I liked your "angel" touch. Your answer cleared my confusion up. Thank you.
Just as a stinger, I'm sending your Christmas gift in the form of pen.