`Diction' refers to the poet's choice of words and expressions and how he uses them to impact the theme.
Regarding theme, the poem deals with the restrictions imposed by the adults upon children's natural freedom, as these stifle their natural growth and development. Blake uses the natural rhythm of speech to convey this theme, particularly as he allows the boy to express himself in his own language, as much as possible. Apart from the rhymed stanzas, most of the lines read like colloquial speech. The following examples illustrate the point. In stanza one, the boy expresses his love of summer in the words: "I love to rise in a summer morn". Then in stanza two, he expresses his unhappiness about going to school in the line," Oh it drives all joy away". Many more lines are couched in such monosyllabic words.
In addition to the above expressions, it can be said that the very words used in the poem are simple. Indeed, the longest word used in the entire poem is "mellowing" which is made up of three syllables. Words associated with spring and summer are used to describe the seasons positively. They include "sing", "joy" "sweet company", "blossom" and so on. The focus of the poem being summer, such words express the boy's delight in being associated with its activities. Similarly, in referring to spring time, he refers to "bud" and "blossom". However, words associated with school do not have positive associations. In school, the boy talks about "sighing" and "dismay" and "dropping". The teachers are "a cruel eye outworn" and school, which should be "learning's bower" has become a "dreary shower", The boy associates his misery with that of a "bird caged" and unable to "sing" or with that of a tender plant that has been "stripped" of its leaves, or with that of "buds that have been nipped before they blossom".
Indeed, there are many more negative words and expressions in the poem. These illustrate the boy's melancholic mood as he laments his inability to enjoy summer because of the insensitivity of his parents to his feelings and natural inclinations by sending him to school. However, beyond this meaning is one larger, which is the need to give children the opportunity to be self-expressive , so they can psychologically withstand the crisis of adulthood. It is to the poet's credit that such profound wisdom is conveyed in the simplicity of the vocabulary or diction.