Learn Spanish (Spain): El libro nuevo tiene noventa y nueve páginas.

In Spanish, numbers from 31 to 99 are written as three separate words connected by y (and), like noventa y nueve (ninety and nine). This is different from the twenties, which are squeezed into a single word like veintinueve (twenty-nine).

Does noventa y nueve need to change to match the feminine word páginas?

No. Most numbers in Spanish don't change gender to match the noun they are counting. The only exceptions in the 1-99 range are numbers ending in one, like uno, which changes to una (for example, noventa y una páginas). Since this ends in nueve, it stays exactly the same.

Why is it libro nuevo instead of nuevo libro?

In Spanish, describing words (adjectives) usually come after the thing they describe. So instead of "new book", you say "book new" (libro nuevo).

Why is there an accent mark on páginas?

Accent marks in Spanish tell you which part of the word to emphasize when speaking. Without the accent mark on the first a, the natural stress would fall on the second-to-last syllable (pa-GI-nas). The accent mark forces the stress to the beginning: PA-gi-nas.