The reader knows before the poem even begins that something terrible is going to happen, thanks to the title. The poem then opens by immediately centering on the "simple soldier boy" and uses the past tense "knew," casting a dark cloud on all the follows.
At first, though, the tone is light and the rhythm jaunty. This reflects the fact that "simple soldier boy" probably signed up for the war with a degree of excitement and anticipation, unaware of the true horrors that were to come. The clear alliteration of the phrase ("simple soldier") emphasizes his innocence and simplicity, adding a sense of lighthearted musicality. The choice of the word "boy" rather than "man" also reminds the reader that this soldier is young.
Indeed, he's probably not seen much yet in his life—which is why he used to "grin at life in empty joy." This implies that his life before the war was a kind of blissful ignorance, more like what life should be for young people.
The enjambment between these two lines flows naturally and easily, reinforcing this picture of pre-war life. But it's interesting that the speaker describes this pre-war happens as specifically "empty joy." Perhaps it was empty because it was missing something fundamental about human life itself—the sheer horrors which humanity is capable of inflicting upon its own kind.
The sound of these two lines reinforce the poem's lighthearted tone, which is at odds with the feeling of dread created by the title. These lines are written in iambic tetrameter, meaning there are four poetic feet with a da-DUM rhythm per line:
I knew a simple soldier boy
Who grinned at life in empty joy,
The clear end rhymes here between "boy" and "joy" add to the sense of ease, making the poem sound so far like a simple nursery rhyme.