Learn Hardcore Romanian: Tu ești un prieten bun.

Romanian is a pro-drop language, which means the verb ending (here -ești) already shows the subject is second-person singular. You can simply say Ești un prieten bun in most contexts. Including Tu adds emphasis or contrast—similar to English “You are a good friend (and not someone else).”

What is ești and how is it formed?

Ești is the second-person singular present tense of the irregular verb a fi (to be). The full present-tense conjugation of a fi is:

eu sunt
tu ești
el/ea este
noi suntem
voi sunteți
ei/ele sunt

What does the article un do in un prieten bun?

Un is the indefinite article for masculine singular nouns in the nominative/accusative case. It corresponds to English “a” or “an.” It tells us we’re talking about one (unspecified) friend.

Why does the adjective bun come after prieten, unlike in English?

In Romanian the default word order for nouns and adjectives is noun–adjective (prieten bun). Placing the adjective before the noun (un bun prieten) is also correct but adds a stylistic or emphatic nuance—much like shifting from “a friend who is good” to the more English-style “a good friend.”

How would you say “You are a good female friend” in Romanian?

You need the feminine forms of the article, noun, and adjective:
Tu ești o prietenă bună.

How would you express this sentence formally or politely?

Use the polite pronoun Dumneavoastră plus the plural verb form:
Dumneavoastră sunteți un prieten bun.
Here sunteți is the second-person plural form used for politeness.

Can the words un prieten bun be reordered, for example un bun prieten? Are both correct?

Yes.

Un prieten bun is the neutral/default order.
Un bun prieten places a bit more emphasis on “good” and mirrors the English “a good friend.” Both are grammatically correct.

How is ești pronounced?

Phonetically it’s /ˈeʃtʲ/. You can approximate it in English as yesht, where ș sounds like the “sh” in “she,” and the final t is slightly softened (palatalized).

What grammatical case are prieten and bun in this sentence?

After a linking verb like a fi, Romanian uses the nominative case for both the predicate noun and adjective. Here prieten is nominative masculine singular, and bun agrees with it in nominative masculine singular.