#15: Für meine Neugier: Stimmt eigentlich der Satz She spoke loud ?
No, not really. People would understand it, of course. Correct would be She spoke loudly, but that sounds overly fussy (pingelig).
You could say she spoke too loud, which is better than she spoke loud, even though the grammar is the same. It's still informal, but it's OK. Or you might say she spoke too loudly, which is also pingelig.
Or: she spoke in a (very) loud voice, which is understandable and standard, not informal.
It's possible to say she ran (too/very) fast, which is fine. I can't explain why she ran fast is OK, and she spoke loud seems awkward.
More generally, some adjectives are frequently used as adverbs in an informal context, such as speech. Most are not. I can't think of any rule or rationale for which are and which aren't. They're along a continuum, where for a specific word, this word usage sounds familiar and OK, or understandable but less common, or not good. I don't know of any rule for this.