It is a copula.

Personally I think that people would be better off thinking of だ and です as completely different things, because other than the most basic uses they aren't really interchangeable and although they can be similar they are actually quite different.
I don't understand why people are taught that they are the same thing.

Both でした and だった express a past tense, but it does not mean that でした is equal to だった.
I'm sorry, I can't give a detailed explanation.

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Edit: All of the above has been said with the assumption that you are thinking they are completely interchangeable.
Sorry if I misunderstood.
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It is a kind of conditional form of the verb おわる
Depending on context it could translate as "If I finish..." or "When I finish..." etc.

-Now has a question-

Someone said だろう is the equivilent of でしょう , is this right?

Depends on how that someone defines "equivalent". If s/he is aware of the difference in the degree of politeness between the two, then I would say s/he was correct. If s/he is saying that they are completely intechangeable, then that isn't the case.

ありがとございます。

We often use "だ" in written language but not so often in spoken language.
There are two major formal styles of writing:

1. "です・ます調" ... Use "です" and "ます" as copula. This style makes polite and gentle impression. This style is used when we write to elders or venders to customers, for example.
2. "だ・である調" ... Use "だ" and "である". This makes serious and explanatory/argument-like impression thus used in such contexts e.g. dictionaries, newspapers and research papers. (The usage of "だ" is similar to "です" but there are some differences Nyagoslav said.)

Normally these styles are mutually exclusive.
Of course there are other rough ways to write Japanese :)

In spoken language, we rarely use bare "だ" because of it's too assertive impression, as some people said.
Instead of it, for example, we use "だね" which implies agreement.

Who is "we"?

I almost never use だ in writing.

I think Japanese normally use in formal context.

Example 1: Newspaper
http://www.yomiuri.co.jp/politics/ne...OYT1T00385.htm
the end of article there's "だ".

Example 2: Research article
"Strong Inference"
This article is written in "だ・である調". there's some "だ"s.

Example 3: Novel
http://www.aozora.gr.jp/cards/000035...237_20060.html
This one is little bit old (because of copyright I can't draw new one by URL) but I can find "だ"s in recent novels I have.

lol You call that "often"?

And what do you figure the chances of OP reading, let alone writing, Japanese newspaper articles in the near future would be? IMHO, One needs to grasp the approximate Japanese proficiency of the questioner in order to answer the question in such a way that it would be helpful to him/her.

I just add other 2 examples.
I think it's sterile to argue about the definition of "often."
The only thing I can say about it is that there's a basic writing style named "だ・である調."

You have ignored all of my questions. :mtongue: