"You do not write a book."
Translation:Tu nu scrii o carte.
Romanian has something which is called "grammatical genders". That means that words are either masculine, feminine, or neutral (neutral nouns simply behave like masculine nouns in the singular and feminine nouns in the plural).
In the example at hand, we have the word "carte" (singular, feminine). The indefinite singular article has two forms: "o" for feminine nouns and "un" for masculine nouns.
Where and how do grammatical genders matter?
Firstly, Romanian does not have a third person singular pronoun for inanimate objects and such (like the English "it") so you will always have to choose between the two gendered pronouns "el" (he) and "ea" (she) when substituting any noun with a pronoun (you can see this sometimes in English as well; for example, sailors that refer to their ship as "she"). The choice of pronoun is dictated by the grammatical gender of the noun in question.
Grammatical genders also bring with them agreement of other words. Articles, adjectives and numerals have feminine and masculine forms (there are some exceptions) and they must match the gender of the noun they modify.
How to determine the grammatical gender of a noun?
You can't. Well, there are some rules (e.g. most nouns ending in "-ă" are feminine), but there are multiple exceptions to them. You simply have to learn the grammatical gender for each noun you encounter. After a while, you will intuitively catch some of the rules and it would become more likely for you to correctly guess the grammatical gender of a given noun.