I have been reading grammar instructions on this website.
When I came across the bottom of the page, there was a quiz. In the quiz it was noted that the word oak in the following sentence is an adjective.
In the spring, red roses blossom in my cute small garden. The beautiful birds also sing in the big oak tree.
Why is oak an adjective in the sentence?
I wouldn't call it an adjective, I'd call it a modifier. A modifier is:
a word or phrase that adds to the meaning of another word or phrase by giving more information about it
A Wikipedia article about modifiers explains that modifiers can be adjectives, prepositional phrases, or other nouns. For example, consider this sentence:
Some loud birds are chirping in the oak tree with the dead branch.
In that sentence:
The word oak is functioning a lot like an adjective – I mean, oak tree looks a lot like yellow car – but I don't think it's entirely correct to call it an adjective.
When parsing a sentence, it can be hard to tell the difference between a noun phrase (like oak tree) and an adjective-noun pair (like hot soup). We can check a dictionary to see if the word in question gets listed as an adjective in the dictionary. For example, a word like compound will have definitions listed a noun and as an adjective, because the word can be used either way. However, most of the dictionaries I checked only listed oak as a noun, so I'd be more inclined to view it as a noun phrase. That said, I wouldn't want to be too dogmatic about the matter.
Oak is not an adjective in this sentence.
The word oak tree is a compound word. This means a word that combines two or more words, but it has the lexical status of being a single word.
Oak tree has its own dictionary entry in the Oxford Dictionary (OED).
Oak tree has its own dictionary entry in the free dictionary.
Third, when you look up oak in the ODO the entry says "also oak tree", meaning this is a variant or that the two are synonyms.
Some compound words are spelled with a space between the two components and some are not; some are spelled with hyphen between the two components. Compound words sometimes start with a hyphenated spelling and after a while the hyphen gets dropped. Some words start as two words and then become so common that they become a compound word (example: Chinese food).
Other compound words that are nouns include
horse race
mind set
silk merchant
car factory
bank clerk
key position
Chinese food
emergency plan
Republican party
rowing boat
energy vampire
punctuality nut
zombie powder
doghouse
bookcase
teapot
blackbird
handlebar
greenhouse
businessman
steamboat
windscreen
sunscreen
sky-scraper
body-blow
bridge-builder
Compound words are different from two words that are, say, an adjective and a noun. The difference can usually be heard in pronunciation.
Compare orange juice (juice squeezed from an orange) and orange juice (any juice that is orange in color). The first, like most compound words, is stressed on the first word and has a falling intonation. The second has a rising intonation on the first word. This distinction in pronunciation is true for the following two pairs, and many similar pairs:
Also key position (the position of a key) as a compound word versus key position (a position that is key or important).
A blackbird is different from a black bird, not only in spelling but in pronunication.
Most these examples are taken from the Concise Oxford Companion to the English Language and The Oxford Dictionary of English Grammar (2 ed.).
Not all collocations are considered compound words. Like most things in English, frequency of usage plays a part. Cough drop and cough medicine are compound nouns; cough trouble is not recognized as one by the OED.
" Oak" is a noun functioning as an attribute in "oak tree".Some people mix the function with parts of speech.
In English, many words can fill more than one function of speech (but of course, many are limited to only one function of speech).
Oak can be a noun or an adjective.
An adjective generally does the job of answering the question "What kind of X?".
While English adjectives can sometimes come after the noun, usually they come before, and adjectives answering "What kind of X" almost always come before the noun.
@NES provided an answer that talked about compound words. Compound words in English can be separated by nothing (e.g. flowerpot), a hyphen (e.g. full-fledged), or a space (e.g. ice cream).
In the case where they are separated by a space, one word is obviously technically a modifier, but combined they mean something different than the original two words. Continuing with the example of ice cream - while ice and cream are used to make ice cream, the end result is not really a type of ice or a type of cream. It's a new thing. You'll probably might occasionally see people spell it incorrectly as one word for this reason, icecream.
Whereas with oak tree - it's still a tree - oak is qualifying the type of tree, thus this isn't really a compound word.
