This is a good book
"To jest dobra książka." or "To jest piękny stół. "will be also ok. Even better.
"To jest książką.", "To jest stołem." - it's, maybe, not gramatically incorrect (and not less correct than "To jest dobrą książką"), but it's just stupid. "To jest dobrą ksiązką." - it doesn't sound SO stupid as in the case before, but I would still say rather "To jest dobra książka".
You may say for example:
Ten samochód jest mercedesem. = This car is a Mercedes.
but you cannot say:
Ten samochód jest mercedes.
you may say, though:
Ten samochód to mercedes.
or just:
To jest mercedes.
or even shorter (but it's rarely used):
To mercedes.
[[[It's a rule in Polish usage of capital letters, that if you use a brand name in the meaning of just a single object, a single thing of the given brand, you should write it with a small letter. It's not often obeyed, especially nowadays, though. The same way, you should write: "Ten panasonic jest zepsuty" = "This Panasonic (for example a TV of the Panasonic brand) is out of order".]]]
When you use "to" with Instrumental, it works so that "to" is a pronoun replacing the subject in the sentence.
"Władca pierścieni" jest dobrą książką. = "The Lord of the Rings" is a good book.*
->
Ta książka jest dobrą książką. = This book is a good book.
(this sentence doesn't look good, normally you would rather say:
Ta książka jest dobra. = This book is good.)
->
To jest dobrą książką. = This is a good book.
(but it still doesn't look good in Polish, it's better to say:)
To jest dobra książka. = It is a good book.
[[[* - again an inclusion about usage of capital letters: in the titles of books, movies etc. only the first letter is in Polish capitalised; BUT in the names of newspapers, magazines etc. all the words begin with a capital letter]]]
In the version with Nominative, the "to" doesn't replace anything. I think it's still considered a pronoun then, from the grammar point of view, but it has, basically, a meaning "this thing".
To order it:
1. Normally after a subject (also a pronoun, like "on", "ona", "oni", "my" etc.) and a form of "to be" - "być" (jest, jesteśmy, jestem, są etc.), when you want to put a noun here, put it in Instrumental. For example:
On jest prawnikiem. -> He is a lawyer.
Ten komputer jest serwerem. -> This computer is a server.
Każdy kwadrat jest prostokątem. -> Each square is a rectangle.
2. "To" is a very special word, whithout which you could live, but it is helpful when you like to use Nominative instead of Instrumental for some reasons. Don't be afraid of it, it's used very frequently.
2a. It can replace a "standard" pronoun of the third person (use it instead on, ona, ono, oni, one), then you put the noun after it in Nominative. Another thing here is that you lose the information about the gender when you do it.
You replace:
On jest prawnikiem. -> He is a lawyer.
with:
To jest prawnik. -> This is a lawyer. (you don't know whether this is a man or a woman)
What is more, while the pronouns on, ona, ono, oni, one are restricted rather for people only, they are rarely used for things, you can use "to" safely also for things.
Having the sentence:
Ten komputer jest serwerem. -> This computer is a server.
you won't rather say:
On jest serwerem. -> It is a server. (it sounds more like: He is a server. - which doesn't make sense in English, and it also doesn't make much sense in Polish, although it makes more sense in Polish than in English, where you virtually never treat non-people as something of a masculine or a feminine gender)
(maybe when someone asks you:
Jaką rolę pełni ten komputer? -> What is the function of this computer?
you can answer shortly, neglecting the subject:
Jest serwerem. -> It's a server.
and this is totally ok, because you don't use the personal pronoun, so there is no problem with that)
It's sounds much better when you say:
To jest serwer. -> This is a server./It is a server. (I don't know which one sounds better in English, I have always problems with that; choose the one which sounds better in English for you :) )
2b. It can replace the verb "to be" - "być" when there is a noun after it, especially in the third person (otherwise it sounds bad). You replace:
Ten komputer jest serwerem. -> This computer is a server.
Każdy kwadrat jest prostokątem. -> Each square is a rectangle.
with:
Ten komputer to serwer. -> This computer is a server.
Każdy kwadrat to prostokąt. -> Each square is a rectangle.
2c. Something in between these two usages is replacing:
To jest prawnik. -> This is a lawyer.
with:
To prawnik. -> This's a lawyer.
It's not used frequently - it's usage is limited mainly to exclaimations. For example:
"Patrz, to Polak!" -> "Look, that's a Pole!"
Or in school primers, because it doesn't demand from the reader to know all the letters of which the word "jest" consists :-) For example:
"To Ala, a to Ola" (Ala and Ola are name diminutives: Ala for Alicja - Alice and Ola for Aleksandra - Alexandra)
It's enough for the child to know the letters: t, o, a and l to be able to read this sentence :-) Try to construct a sentence with the word "jest" that would use only the letters: j, e, s and t. Rather impossible.
Oni są PolKIMI. - Females only! = They are Polish.
One są Polkami.
or with "to":
To są Polki.
or:
To Polki.
also in such a way it wouldn't be very bad (but I would definitely avoid it):
One to Polki.
There is no rule that for children I would use "Oni są Polakami" and not "One są Polkami". "One są Polkami" for women/girls only (regardless of their age), and for other cases "Oni są Polakami".
