Taking these classifications from Oxford's Lexico:
Why is “brick” in “a brick house” a noun, whereas “plastic” in “a plastic bucket” is an adjective
adjectivesnounsparts-of-speech
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This is just headliner confusion. British is virtually always an adjective, not a noun. The noun is Britain. But headlines of late have been using the shorter noun forms, as in Spain court in lieu of Spanish court. I wouldn’t recommend it outside of headlines, if even there.
Be that as it may, we aren’t going to start calling British, French, German, Spanish, Chinese, or Japanese “nouns” just because their corresponding noun forms are now getting used as noun-adjuncts in headline English.
One example of British as a noun might be:
We haven’t seen many British around here lately.
Although another is:
We haven’t seen many Britishers around here lately.
Neither of those is particularly well thought of. In writing, Briton is usually preferred:
We haven’t seen many Britons around here lately.
But that can of course be misheard in speech as:
We haven’t seen many *Britains around here lately.
Which is one reason why it tends to be shorted to the casual:
We haven’t seen many Brits around here lately.
Substitute is indeed a noun:
substitute n.: One that takes the place of another; a replacement.
As @EdwinAshworth points out, it is also an adjective:
being such in appearance only and made with or manufactured from usually cheaper materials; synonym: artificial - MW
Regardless of the label, it functions as an adjective in the pair substitute teacher.
Replacement is a noun, and means substitute as well. This is a case of a noun acting as an adjective. Replacement teacher is exactly the same as substitute teacher. Does it retain its essential property as a noun when used as an adjective?
According to Grammar Monster, most compound nouns are made up of two nouns or an adjective and a noun. For example:
Noun + Noun: Bath tub, witchcraft, seaman, wall-paper
Adjective + Noun: Hardware, highway, full moon, whiteboard
Other sites define it similarly, or restrict it to two nouns. Edufind Is even looser, stating it's a noun plus another word and gives examples:
noun + noun: bedroom
noun + verb: rainfall, haircut
noun + adverb: hanger-on, passer-by
verb + noun: washing machine, driving licence
verb + adverb: lookout, drawback
adjective + noun: greenhouse, software
adjective + verb: dry-cleaning, public speaking
adverb + noun: onlooker, bystander
adverb + verb: output, upturn, input
But this we do know: technically, Adj. + Noun => compound noun. And, noun + noun => compound noun. So, I think that is what the comments above are about. Labels may complicate things. There are lots of labels for the same thing. You do not need to know all the labels to learn, write, and speak English.
A substitute teacher is the same as a replacement teacher. Until the replacement becomes permanent. In this economy, both are a blessing.
Edited to reflect information in comment.
Best Answer
This may have been addressed here before, but the overall answer is 'there is no consensus as to when certain words should be considered attributive nouns, and when they should be considered to have converted fully to adjectives', obviously in these instances used prenominally. 'Steel bridge' is a famous case in issue.
Nordquist at ThoughtCo discusses this issue:
The usual tests for adjectives include gradability and intensifiability, but just as 'steel' in 'steel bridge' fails this
so does the obvious classifying adjective 'nuclear'
For particular classifications, only asking say the compilers why they chose contrasting POSs for apparently identical usages will begin to resolve the question.
But checking in the usual respectable dictionaries (in particular AHD, Collins, RHK Webster's, Lexico, M-W, CD, Longmans, Macmillan) for POS assignment of 'plastic', all concur that full conversion to the adjective has now occurred, even for the basic 'made of plastic' ('plastic spoon') sense.
But for 'brick', Merriam-Webster has 'noun, often attributive', Collins calls the usage 'noun as modifier', and Lexico and Macmillan also list 'a brick wall' under [noun]. CD is, I'd say, unclear (though gives 'red-brick houses' under [noun]). AHD and Longman seem not to address the issue, but do not list adjective usages. However RHK Webster's classifies 'brick' in the senses 'made of, constructed with, or resembling bricks' [adjective].