Learn French: Ce que tu bois a une saveur délicieuse. - What you are drinking has a delicious flavor.

In French, ce que is used when "what" (meaning "the thing that") is the direct object of the following verb. Here, you are drinking it, so ce que is the object of bois. If "what" were the subject performing the action, you would use ce qui instead.

Could I use quoi instead of ce que to say "what you are drinking"?

No, quoi is mainly used for asking questions (like Tu bois quoi ?) or after prepositions. When "what" means "the thing that" to connect ideas in a statement, you must use the indefinite relative pronouns ce que or ce qui.

What is the subject of the main verb a (has) in this sentence?

The subject of a is the entire clause Ce que tu bois (What you are drinking). When a phrase like this acts as the subject of the sentence, the main verb is always conjugated in the third-person singular, which is the il/elle/on form.

Why does the adjective délicieuse come after the noun saveur?

Most descriptive adjectives in French go after the noun, especially those describing sensory details like taste, color, or shape. Notice also that délicieuse is in its feminine form to agree with the feminine noun saveur.