They can be identified by these characteristics:
1. They are not necessary to make the sentence complete.
2. They can be located anywhere in a sentence, beginning, middle, or end.
3. They are most often punctuated with commas.
4. They are usually identified by the first word in the phrase.
There are seventeen kinds, falling into six groups, namely, one each for five parts of speech (noun, verb, adjective, adverb, preposition [the other three parts of speech—pronoun, conjunction and interjection—aren't usually used in free modification]) and one comprising five types of clause:
- verbal clusters: present participial, past participial, infinitive [= verbal + modifiers or a noun], as is, detached (Jim opened the door, but later closed it)
- nominal clusters: noun (A skilful burglar, Jim opened the door [a.k.a. an appositive]), list, such as
- adjectival clusters (Curious about the noise, Jim opened the door) [= adjective that emphasises the noun]
- adverbial clusters (Unfortunately, Jim opened the door) [= adverb that emphasises the noun]
- prepositional phrases (Like a butler, Jim opened the door)
- altered clauses: subordinate, free relative (which, for which), free absolute (His fingers trembling, Jim opened the door [= noun + free modifier]), free that (His goal—that the door be open—was achieved), quote-attributing clause
Note also resumptive (He finally faced his biggest fear, a fear that had plagued him since he joined the team) and summative (He finally faced his biggest fear, a debilitating obstacle that had plagued him since he joined the team) modifiers [N.B. they can modify any kind of word, phrase or clause].