Can someone finally explain the difference between "laufen" and "rennen" please? Both were used alternatively for "running", but there must be some meaning/context difference, right? Here "laufen" is suddenly walking (taking a stroll?) a.k.a. "wander(e)n" or "spatzieren" or "gehen" ...
I am not aware of any sources that list Garner's Modern English Usage, first published in 1998, as an authoritative dictionary. The Oxford English Dictionary defines "safe" as an adjective and a noun, but makes no mention of it as an adverb:
Admittedly, the OED is British, so one could make a case that this is an American colloquialism, but Webster's, typically cited as the definitive American dictionary, also has no definition for "safe" as an adverb:
The only mention of an adverb there is under "Other words from safe", indicating that usage of "safe" in the sense of an adverb is an "other" case, i.e. not defined in the dictionary and mentioned as a fringe usage.
Speaking of Merriam-Webster, there is another article on their site discussing exactly this usage, which it notes is called a "flat adverb":
As that site notes, however, use of flat adverbs used to be more widespread in older literature, and have gradually filtered out of modern usage. So contrary to possible claims that flat adverbs are a modern or colloquial usage, it turns out that precisely the opposite is true: Flat adverbs are an archaism which do not reflect either prescriptive or modern English. Yes, obviously some of them survive in the colloquial language, but dictionaries do not reflect this usage as correct, and history suggests this usage is obsolete.
"Slowly" is an adverb, the key is the second part of the word: adVERB. Adverbs are called so, because they are used with verbs. "To walk" is a verb, so it requires an adverb. Adjectives, on the other hand, are used with nouns as a description, or by themselves. For example: "I have done the job" -"Good"