Learn Hardcore Romanian: Eu citesc o carte bună.

The pronoun eu (“I”) marks the subject explicitly, but it isn’t strictly necessary because Romanian verbs are already conjugated for person. Citesc is first-person singular, so Citesc o carte bună also means “I am reading a good book.” Including eu adds emphasis or clarity.

What tense and person is citesc, and how do you form the present tense of a citi?

Citesc is the present indicative, first-person singular of a citi (“to read”). The full present-tense conjugation is:
eu citesc
tu citești
el/ea citește
noi citim
voi citiți
ei/ele citesc

What role does o play before carte, and why not un?

O is the feminine singular indefinite article (like English “a/an”). You use un before masculine (and neuter) singular nouns and o before feminine singular nouns. Thus o carte = “a book.”

Why is the adjective bună placed after carte, and why does it end in -ă?

In Romanian the normal order is noun + adjective: carte bună. The adjective must agree in gender and number: carte is feminine singular, so bună takes the feminine singular ending -ă. For a masculine noun you’d say, for example, un băiat bun.

Why isn’t there a pe before o carte bună to mark the direct object?

Romanian uses the particle pe to mark definite, animate, or highly specific direct objects (especially people). Here o carte bună is indefinite and inanimate, so you simply place it after the verb without pe.

How would you say “I read the good book” with the definite article?

Attach the feminine singular definite article -a to carte: cartea. You get: Eu citesc cartea bună (or simply Citesc cartea bună) = “I am reading the good book.”

How do you pronounce the letter ă in bună, and how does citesc sound?

The letter ă is a schwa [ə], like the ‘a’ in English “sofa,” so bună is [ˈbu.nə]. Citesc is pronounced [ˈt͡ʃi.tesk], because c before i gives a [t͡ʃ] (“ch”) sound.

How would the sentence change if you were talking about multiple books?

For an indefinite plural you use niște + plural noun + plural adjective:
Eu citesc niște cărți bune.
(“I am reading some good books.”)
For a definite plural you add the article -le to the noun:
Citesc cărțile bune.
(“I am reading the good books.”)