Let's take two example sentences:
Вот пара ребят. Они братья.
and
Вот пара предложений, которые я выписал в тетрадь. Это упражнения. Я выписал их из учебника.
Why can't we say:
Вот пара предложений, которые я выписал в тетрадь. Они упражнения. Я выписал их из учебника.
In Russian like in English, you can use two kinds of constructions to name an object, "He is a man" and "This is a man":
He is a man. - Он мужчина. [1]
This is a man. - Это мужчина. [2]
The difference is, construction [2] in Russian will always begin with это irrespective of the number of the objects:
This is a man. - Это мужчина. [2]
These are men. - Это мужчины. [2]
Also, in Russian, you cannot use construction [1] to name non-living things and most animals, too. In this case in English you will have it/they in construction [1], in Russian it will again be это irrespective of the number of the objects:
It is a sentence. [1] - Это предложение. [2]
They are sentences. [1] - Это предложения. [2]
Consider the following:
Вот пара предложений, которые я выписал в тетрадь. Это <есть> упражнения. (They are exercises)
Вот пара предложений, которые я выписал в тетрадь. Они <есть> короткие. (They are short)
Although the personal pronoun они is perfectly correct in the latter case, you will predominantly encounter this usage where a quality (or an action) of the subject for which the pronoun substitutes is described. The former case, on the other hand, tends to be used where the substituted subject is (re)defined. This usage of это is so widespread that this demonstrative pronoun is said to not only reply to the questions Какое? (where it is translated as this) but also to the question Что это? (where it then means it (is)).
In other words, in constructs like это упражнения the pronoun это serves as a personal pronoun rather than a demonstrative one. As has already been said, in its personal pronoun incarnation это is much more often substituted for inanimate, rather than animate subjects and its plural form coincides with its singular one.
Видишь этих людей? Они инженеры. - Do you see these people? They are engineers.
(Это инженеры would be also perfectly correct, however!)
Видишь эти предметы? Это стулья. - Do you see these objects? They are chairs.
(Они стулья would sound terribly weird as if someone cursed people into the shape of chairs)
I hope this helps.
Well, those examples are quite strange indeed. I suppose those sentences have some connection, otherwise the first example wouldn't have much sence. Your second example is grammatically correct but it is something like:
I stare at the golden coins on my table. **These are** the coins. I've put them on the table.
Sounds like a talk of a slightly mad man. And the third example is total nonsence, it is like:
I stare at the coins on my table. **They are** the coins...
I don't believe you can say "they are the coins" in English. In Russian it's similar, also you don't have to distinguish between singular/plural, so in Russian it is just like if you literally could say "it's a coin" - "это монета" or "it's coins" - "это монеты", so it is even simpler than in English.
