What is the tone of the speaker in Dylan Thomas' poem "Fern Hill"? What does the speaker lament?

Generally, the tone of a speaker is found by closely examining the diction and syntax in a piece of writing. In Dylan Thomas' "Fern Hill," studying the diction in the poem (the word choice, in other words) should clearly show you that the tone is idealistic and nostalgic.

"Fern Hill" is essentially a look back at childhood, and most of the language is playfully infused with a sense of magic. Take, for instance, the first five lines of the poem:

Now as I was young and easy under the apple boughs

About the lilting house and happy as the grass was green,

The night above the dingle starry,

Time let me hail and climb

Golden in the heydays of his eyes... (Thomas 1-5)

Notice the nostalgia inherent in phrases like "happy as the grass was green" and "Golden in the heydays of his eyes." Additionally, notice the playful diction, such as "lilting house" and "the dingle starry." Thomas' diction signals two things: first, this is an examination of a child's perspective, as the word choice clearly reflects a child's perception of the magic inherent in everyday life. Second, this perspective is viewed through a lens that is primarily nostalgic for the loss of a magical childhood.

Accordingly, it's apparent that the speaker laments the loss of childhood, and it's clear that there is a certain vibrant quality that is lost once childhood passes. As a whole, the poem is largely also lamenting the passage of time, as is evidenced by the last few lines: "Time held me green and dying / Though I sang in my chains like the sea" (44-5). In this passage, it is obvious that Thomas is reflecting on the passing of time with sadness and yearning for days of happy and carefree youthfulness.