Because libro is a masculine singular noun that begins with a normal consonant, so it takes un.
Italian indefinite articles:
Examples:
So libro correctly takes un.
Because in Italian, adjectives often come after the noun.
So:
This is very normal in Italian. English usually says:
But Italian commonly says:
Some adjectives can come before the noun in Italian, but nationality adjectives like inglese normally come after it.
It can mean both, depending on context.
In this sentence, un libro inglese usually means:
Elsewhere, inglese can also mean:
So the exact meaning comes from context.
Good question. Some Italian adjectives have the same form for masculine and feminine singular.
Inglese is one of those adjectives.
Singular:
So inglese stays the same in the singular for both masculine and feminine, but changes to inglesi in the plural.
Yes, and that is also correct, but the structure is different.
The first identifies what this is.
The second starts with this book and then says something about it.
In many situations, both are possible, but they are not exactly the same sentence pattern.
Because inglese here is an adjective describing libro, not a noun by itself.
Here, un goes with libro, and inglese simply describes the noun.
But if inglese is used as a noun, you may need an article depending on the sentence:
So in your sentence, no extra article is needed before inglese.