At its most basic, a poem is a scattering of black marks on a page. The reader has to restore to these anonymous little squiggles something of their lost material body. Poets are materialists of language, and readers have to become the same. We have to grasp these marks as tonal, metrical, rhythmical, emotional and expressive of meaning. To do this, we must bring to the poem a whole framework of cultural knowledge.

Take the question of tone. Strictly speaking, there is no such thing as toneless speech. We hear language as obstreperous or obsequious, genial or plaintive. Verbal tones can be arch, abrupt, dandyish, urbane, lugubrious, exhilarated, autocratic.

Tonelessness itself has an expressive significance. As with robots, Daleks, psychopaths and mad scientists it suggests a sinister lack of human sympathy. Tone lies at the intersection of words and emotion. It is one way in which the body — feelings, reflexes, responses — gets into language. “Mood” is another. To be tone-deaf in human intercourse is a fearful disability.

A shift of tone may signal a shift of meaning. Your “good morning” may be frosty or fawning. There is a word-game in which two or more people take a single monosyllabic obscenity and give it a different tonal inflection each time. The result isn’t quite as grand as Bleak House, but it makes a point all the same — you can tell a kind of story by changing your tone.

How do we identify tones? We are all born into a specific culture. We learn tones and moods along with tastes, smells and concepts. Face-to-face, identifying tones can be straightforward. We know she is being sarcastic when she cries “I’ll never desert you!” because, as she says it, she is clinging amorously to someone a lot smarter, kinder and sexier than you. With poems things are trickier. Poems do not come marked F major or B minor. Because there is no face-to-face context to determine exact significance, poetic language is bound to be more ambiguous than speech. What you hear as absurdly grandiloquent is passionately persuasive for me. One reader’s garrulousness is another’s eloquence. But there are limits to this ambiguity. Take these exultant lines of W. B. Yeats: When such as I cast out remorse So great a sweetness flows into the breast We must laugh and we must sing, We are blessed by everything, Everything we look upon is blest.

If someone really heard this as cantankerous or dispirited, we might ask how well they understood English. Tone, mood, pitch, pace and timbre are not just “subjective”, since they are closely bound up with meaning. And there can be no such thing as private meaning.

(One can even speak of the “volume” of a poem. Some bellow lustily, some are low-keyed, some toll melancholically like muffled bells, while others murmur so shyly that one has to bend close to catch what they are saying). It is not true that ideas are public whereas feelings are private. Poetry is one refutation of this.

What, then, of mood? We can describe this as part of a poem’s peculiar “sensibility” — an emotional pattern generated by its language. Tennyson is a past-master: With blackest moss the flowerpots Were thickly crusted, one and all: The rusted nails fell from the knots That held the pear to the gable-wall. The broken sheds looked sad and strange: Unlifted was the clinking latch; Weeded and worn the ancient thatch . . .

It’s unlikely that you would land a first in your English Finals by reading this as uproariously funny. Yet neither would you deserve a first if you simply pointed out how listless, enervated and melancholic it is. I would give top marks to a candidate who argued that the mood is too obvious and obtrusive. Every detail is dragooned into the emotional effect. Nothing is present by chance, nothing can get away. Even the nails fall obediently from the walls. The mood is far too coherent.

Notice, too, how the repetitive, undynamic rhyme scheme contributes to the overall stagnation and inertia. A student able to write about the poem in this style would be well on the way to wielding the tools of the trade.

How to Read a Poem is published by Blackwell (£9.99, offer £9.49 inc p&p from 0870 1608080)

Times Recommends

• The 100 Best Books of the Decade
• The Times picks the 50 best paperbacks of 2009
• The conversation: James Ellroy