Learn Arabic (Modern Standard): أنت ذكي حقا!

The base word is حق (truth or right). When we want to use it as an adverb meaning "really" or "truly," it takes the indefinite accusative case. For most words, this case is marked by adding an Alif with a double-Fatha sound (ـاً), which is exactly what happens in حقا.

Does حقا follow the same rules as degree adverbs like جدا?

Yes! Just like جدا (very) and كثيرا (a lot), حقا is an adverb frozen in this accusative state. Notice how it is placed directly after the adjective ذكي (smart) that it describes, which is the standard word order in Arabic.

Where is the verb for "are" in this sentence?

Arabic does not use a present tense verb for "to be" (like am, is, or are). You simply place the pronoun أنت (you) right next to the adjective ذكي (smart) to create the complete thought "You are smart."

If I want to say this to a woman, do I need to change حقا to be feminine?

No, adverbs like حقا never change for gender or number. However, you do need to change the pronoun and the adjective! You would say أنتِ ذكية حقا! using the feminine pronoun أنتِ (anti) and adding a Ta' Marbuta (ـة) to make the adjective ذكية (smart) feminine.