“The water-resistant shoes are great for rainy days.” “The table is rock hard.” In these sentences, I know “water resistant” modifies shoes and rock hard modifies table. Does “hard” modify “rock” and does “resistant” modify “water?” I am confused on this.
The answer is that the first word in the compound adjective is sometimes a modifier, sometimes a complement, and sometimes changes the meaning completely. There is a brief discussion of this in the Cambridge Grammar of the English Language p1656-1657:
Noun + adjective compounds
The majority of compounds with an adjective as second component have a noun as the first. In general, there is no contrast here between a compound and a syntactic construction since adjectives take only a highly restricted type of NP as pre-head dependent (cf. Ch. 6, §3.2): the syntactic dependents of adjectives are generally pre-head adverbs or post-head PPs and clauses. Many noun + adjective compounds involve a high degree of lexicalisation, as in:
 colour-fast, foot-loose, headstrong, threadbare, top-heavy
Although there is a more or less obvious connection between the meaning of the whole and that of the adjective head, none of these satisfy the test for hyponymy. He is headstrong, for example, does not entail He is strong, and something can be top-heavy even though it is as a whole relatively light. We will not attempt a comprehensive review of the patterns to be found, but will illustrate a selection of the more productive ones.
bone-dry, crystal-clear, dirt-cheap, dog-tired, feather-light, ice-cold, paper-thin, razor-sharp, rock-hard, stone-deaf
bottle-green, brick-red, jet-black, snow-white, steel-blue
Here the noun indicates a standard of comparison: “dry as a bone”, “clear as crystal”, etc. Very often, as in [i], the effect is to intensify: bone-dry means “extremely/completely dry”, and so on. A special case of the comparative type is that of colour adjectives, as in [ii]; jet-black and snow-white are intensifying, but the others simply specify a particular shade of the colour. Compounds of this type are clearly hyponymic: if you are dog-tired, then necessarily you are tired, and so on.
ankle-deep, shoulder-high, skin-deep, state-wide, week-long
This is a productive pattern, with the noun indicating extent. Wide here has to do with area rather than the one-dimensional measure denoted by wide on its own, and skin-deep “superficial” is a further example of lexicalisation. Compounds formed on this pattern are non-hyponymic: The water was ankle-deep, for example, does not entail The water was deep. We noted earlier that there may be a variety of reasons why a compound might fail the hyponymy test: in the present case it is due to the fact that the adjectives are gradable ones that can apply either to the scale generally (How deep is the water?) or to an area of the scale greater than some relevant norm (The water is deep). The compound involves the first use, whereas the adjectives are generally interpreted in the second way when standing alone.
accident-prone, burglar-proof, camera-shy, carsick, cholesterol-free
class-conscious, girl-crazy, oil-rich, power-mad, praiseworthy
snow-blind, tax-free, travel-weary, user-friendly, watertight
These are comparable to syntactic constructions where the adjective has a following PP as dependent, complement, or modifier - compare prone to accidents, proof against burglars, crazy about girls, rich in oil, etc. Free (both in the sense “not having to pay”, as in tax-free, and in the sense “not containing”, as in cholesterol- free) is particularly productive. Some adjectives, such as crazy, free, mad, rich, weary, worthy, occur readily both in compounds and in syntactic constructions, while others, such as proof and tight, prefer or require the compound form. Others again take syntactic complements but hardly form compounds: fond of animals, keen on sport, eager for revenge (compare *animal-fond, *sport-keen/*sports-keen, *revenge-eager). Where the noun corresponds to a syntactic complement, the compounds are generally not hyponymic: tax-free goods aren’t (necessarily) free, nor is a user-friendly computer manual a friendly one. With prone and proof the issue does not in fact arise since they cannot stand alone without complements - and indeed the same applies to free and conscious in the senses they have in cholesterol-free and class-conscious.
self-confident, self-concious, self-evident, self-important, self-righteous
There are a great many adjectives with self as the first component; many belong in the verb-centred category (self-denying, self-declared), but there are a considerable number which are adjective-centred, like those in . A high proportion apply to humans (but cf. self-evident, self-contradictory). A few are hyponymic (self-confident, self-contradictory), while others are clearly not (self-important, self-righteous).
The above are sometimes found without hyphens.
Indeed, a review of accident records for major airline equipment in the past few years seem to indicate that GE and Airbus are no more accident prone than others, such as main competitor engine-maker Pratt & Whitney. (San Francisco Chronicle; 2001-11-13)
The week long operation ended on Thursday. ICE officers in L.A. reportedly just finished a similar operation. (Los Angeles Times; 2019-07-13)
The same spectra of Ka and Fk were seen in the work of Koren (1999), who pointed out that this brick red dye (Ka), which was extracted from textiles, is found only in an oak-kermes coccoid scale species and is not present in any plant or cochineal insect. (ACAD: Bioscience; Dec2005, Vol. 55 Issue 12)
He didn't feel called on to mention the extra vote for himself the widow would bring into the Beat. Nor his anxiety that Woodrow, being too foot loose and fancy free, might light out for the bottom and never put a vote in a ballot box. (FIC: The Mississippi Quarterly Fall 2001; The Best Laid Plans; John Faulkner)
Phrases like "rock-hard" and "water-resistant" are a special subclass of idioms in English (and other Germanic languages: cf. German "wasserdicht", "feuerfest"). Sometimes they are single words ("fireproof"). They are a strange kind of idiom because their meaning is not metaphorical (as with most idioms) but is not entirely determinable from merely the sum of their parts either (as with phrases that aren't idioms). "Water-resistant" is a compound adjective and, as an adjective, modifies nouns. It represents a kind of "shorthand" for "resistant to water", i.e. the shoes are resistant (adjective) and they are resistant "to water" (where "to water" would, grammatically speaking, be an adverb describing how, or in what way, something is resistant).
Because these types of phrases (like "resistant to water" or "proof(ed) against fire") are so common, in Germanic languages at least (not sure about other language families) they get "shortened" to a compound adjective in which the noun from the original adverb phrase ("to water") gets shortened and stuck on to the beginning. The general rule is to hyphenate the compounds if they consist of two or more elements and appear before a noun, and otherwise not to hyphenate.
Neither, As in your first usage "water-resistent", they should be hyphenated, The act as a single word adjective. From grammerly.com
A compound modifier is made up of two words that work together to function like one adjective. When you connect words with the hyphen, you make it clear to readers that the words work together as a unit of meaning.
If you left off the hyphen the meaning is still that of a single word adjective.
Great question, and the short answer is yes, they modifier the modifiers while still acting as a single unit to modify the subject. By the way you worded your question, I can tell you understand the concept of compound modifiers, but I'll still include the information on them below in my answer - since others may have this same question while not possessing your understanding of the concept. I don't have sources written down on this anywhere, but I'm sure I could find some if you need it. I've taught English Language Arts for about 16 or 17 years, and I love studying both linguistics and grammar.
Something important to remember as that the function of words, phrases, and clauses can be different than their individual parts of speech. For example, "hard" can be an adverb when "my students work hard" because it explains how they work. It can also be an adjective when "students think my class is hard". It can also be a compound modifier when "Hard-working students are often rewarded."
In your example sentences. You have two different grammatical constructions and they both use compound modifiers (one adjectival phrase working to modify a noun or pronoun). Your compound modifiers are each made up of adverbial nouns (nouns or noun phrases that behave like adverbs and adjectives) and adjectives.
The first one, "The water-resistant shoes are great for rainy days.", uses the compound modifier (water-resistant) to modify the subject (shoes). The adverbial noun (water) modifies the adjective (resistant). Together, they modify the subject (shoes).
For the above sentence, here are the functions:
- "Shoes" = Subject
- "water-resistant" = Modifying the subject (shoes). Water resistant is behaving as a single modifier here.
- "are" = linking verb. "are" links the subject (shoes) to the complement (great).
- "great" = complement (LVCA). "great" is a complement that completes the linking verb (are).
- "for rainy days" = Prepositional Phrase. "for rainy days" is a prepositional phrase made up of the preposition (for), a modifier (rainy), and the object of the preposition (days).
- "The" = article pointing to shoes.
The current construction allows "water-resistant" to modify shoes.
The second construction, "The table is rock hard", also makes use of a compound modifier to modify the subject. The difference is the compound modifier comes after the noun it modifies and doesn't need a hyphen (like water-resistant shoes). For the above sentence, here are the functions:
- The = Article pointing to the subject (table)
- table = Subject
- is = Linking Verb connecting the subject and compound modifier
- rock hard = compound modifier describing the table and completing (LVCA) the linking verb.
The current construction also allows for both words to behave as one modifier and describe the table.