I answered "habla más alto" to a "write what you hear" form of this question. Duo said I incorrectly used the the command form "habla" instead of the 3rd person you form "hable". The conjugations I have of hablar show the formal 3rd person present of hablar as "habla," not "hable." Can anyone explain why Duo says it's hable here?
Habla is the present tense form for él, ella, usted. It is used to tell what happens or is happening. Señorita, (usted) habla español muy bien. Miss, you speak Spanish very well.
Hable is the imperative for usted. It is used to give a formal command. Señorita, hable (usted) más alto, por favor. Miss, speak louder, please.
Habla is also the imperative for tú. It is used to give a familiar command. Carmen, habla (tú) más alto, por favor. Carmen, speak louder, please.
An adjective is used to modify a noun, pronoun, or another adjective. An adverb is used to modify verbs or other adverbs. "Speak" is a verb, so "Speak more loudly" is grammatically correct. "More loudly than you currently are," is understood. "Speak louder" may be heard, but it is really not correct since the adjective "loud" should not be used to modify a verb.
Actually, adverbs modify verbs, adjectives or other adverbs. Adjectives do not modify other adjectives. :)
Comparative adjectives like "Louder" can, and do function as adverbs. [He speaks louder than his opponent] [Speak louder!]
More Loudly is likewise correct but is usually heard less often. [He speaks more loudly than his opponent.] (particularly in command form) [Speak more loudly!]
"Speak more loudly" is definitely correct, since the comparative form of the adverb 'loudly' is 'more loudly' or, apparently (I checked a couple of dictionaries), 'louldlier'. If all these options sound weird to you (and they do to me), that is because they are rare, especially 'loudlier' (even my spellchecker is not happy with that word). The word 'loud' is an adjective AND an adverb, and its comparative form (for both parts of speech) is 'louder'. So it is absolutely correct (and natural for many, including myself) to say "Speak louder."