Sentence Correction (SC) | Re: I am struggling with noun and verb modifiers, especially

Some words can be used as both adjectives and adverbs; flat is one among them. A few more are late, fast, hard, close deep etc. These do not need to be suffixed with the 'ly' tag to denote that they are adverbs.

The test is to find the answer for these question, - packed when, packed where, packed how and some such. If the word could answer any one of the questions, then it is an adverb, since adverbs denote time, manner, place etc.