Adjectives That Only Work in Front of the Noun

Most adjectives go in either position without complaint:

a red door / the door is red
a difficult question / the question is difficult

Some do not:

a mere formality / the formality is mere — wrong
the main reason / the reason is main — wrong
an utter disaster / the disaster is utter — wrong

The two positions

An adjective in front of its noun is attributive. An adjective after a linking verb is predicative. Most English adjectives are comfortable in both. A small number are restricted to one, and the restrictions are not random.

Attributive-only: the four groups

Intensifiers. Words whose job is to strengthen or weaken the noun rather than describe it: mere, utter, sheer, absolute, complete, total, outright, downright. An utter disaster means a very great disaster; utter is not a quality the disaster possesses, so there is nothing for a linking verb to link to.

Ranking and position words. Main, chief, principal, primary, former, latter, only, sole. These locate the noun among other things rather than describing it.

Classifying adjectives. Nuclear physics, atomic energy, criminal law, medical school. These sort the noun into a category. The law is criminal is a grammatical sentence with a completely different meaning — it means the law is a crime.

Some adjectives of degree with people. A big eater, a heavy smoker, an old friend. These modify the activity, not the person. My friend is old is fine but means something else entirely — and that shift is the clearest evidence that the attributive version was never describing the person at all.

Predicative-only: the other direction

A smaller group works only after a verb. Many begin with a-:

afraid, alone, alike, alive, asleep, awake, aware, ashamed, ablaze
content, glad, ready, sorry, unable, well

The child is asleep is fine; the asleep child is not. English reaches for a different word instead — the sleeping child, the frightened dog, the living relatives.

The a- prefix here is a fossil. It descends from an Old English preposition meaning roughly in or on, so asleep was once closer to in sleep — a prepositional phrase, which explains why it still resists sitting in front of a noun a thousand years later.

The ones that change meaning

The most interesting group works in both positions and means different things in each:

an old friend (long-standing) / my friend is old (elderly)
a poor student (weak at studying) / the student is poor (has no money)
the late Mr Hall (deceased) / Mr Hall is late (has not arrived)
a certain outcome (particular, unspecified) / the outcome is certain (guaranteed)
a present member (current) / the member is present (here)
a heavy drinker (drinks a lot) / the drinker is heavy (weighs a lot)

The late Mr Hall is late is a perfectly grammatical sentence in which the same word means two unrelated things eight words apart.

Why this matters in practice

For native speakers, mostly as a curiosity. You will not write the formality is mere by accident.

It matters when learning English, where the restriction is invisible and the errors are impossible to hear yourself make. It matters in translation, where a source language may allow both positions freely. And it matters when the meaning shifts: a poor student in a school report is a sentence worth being certain about.

The test

Move the adjective after is and see whether it survives. If it does not, the adjective is doing something other than describing — intensifying, ranking, or classifying. If it survives but means something new, you have found one of the interesting ones, and the position you choose is now a decision rather than a preference.