"The men and the women are powerful."

Translation: Vali ābrī kostōbi issi.

[V8wz6]

what is the difference between kostõbi and kostõbī? are they not both plural of strong?

[Ctxkh1Me]

They're different cases. For Lunar nouns such as vala and ābra, kostōbi is the nominative plural form. The nominative case is what we use with the subject of the sentence. Since the verb is issi, that's also what we use for kostõba in this sentence. In the accusative plural form, we use kostõbī, e.g. vali kostõbī ābrī jorrāelzi "the men love the strong women."

You can read about the various noun cases here. For more on how adjectives are modified, check out https://wiki.dothraki.org/High_Valyrian_Adjectives