Gerard Manley Hopkin's poem "God's Grandeur" is written in "sprung rhythm," a term Hopkins himself coined.
The poem is broken into two stanzas, an octave followed by a sestet, and comprised of fourteen lines total. Each line is made up of ten syllables, except for line three which contains twelve. The pattern of stressed and unstressed syllables, however, is not just a basic iambic pentameter. Rather than a stressed syllable always following an unstressed syllable to create lines with five iambs each, in Hopkin's poem the poet allows this meter to break and change in order to best express his meaning. It's a meter that has been freed, or "sprung" in Hopkins words, from strict rules. Though the poem frequently rests in iambic pentameter, the use of sprung rhythm allows for significant breaks, as in line three's ending "shining from shook oil" and line's five opening word "generations."
Overall, the poem is in iambic pentameter, but there are some irregularities in the first five lines.
An iamb is a poetic foot consisting of an unstressed syllable followed by a stressed one. In iambic pentameter, each line has five iambs in a row. This is a very natural meter for English.
Most of the poem "God's Grandeur" can be scanned in iambic pentameter:
And though the last lights off the black West went,
Oh, mor-ning, at the brown brink east-ward, springs --
Read the the bold syllables as stressed and these lines will fall naturally into iambic pentameter.
The first few lines of the poem do not work this way. The first line contains two iambs followed by two anapests. (An anapest is a foot with two unstressed syllables followed by one stressed one.)
The world is charged with the gran-deur of God.
Anapests usually give a line a jaunty feel, as if it is moving along quickly. In this case, it gives the poem a conversational feel and gives us the sense that the poet is charging headlong into the poem, eager to tell us about God's grandeur.
Line two can be read to contains five iambs, but line three departs from iambic pentameter again, this time with six iambs. The extra beat in line three gives the line a statelier feel which matches the idea of something "gathering to a greatness."
It gath-ers to a great-ness, like the ooze of oil
Line four has five feet, four of which are iambs, but the first one ("Crushed. Why") is a spondee, a foot in which both syllables are stressed. The spondee brings the reader up short, preparing us for the cacophony that comes in the rest of the line as well as the frustration it is expressing.
Line five begins with the awkward word "generations." This forces the line to be scanned as beginning with two anapests, followed by two iambs:
Ge-ne-ra-tions have trod, have trod, have trod,
We can almost hear treading feet in the line (slowing down with weariness toward the end, as they move from anapests to iambs), and the fact that the line has only four feet instead of five creates a sense of urgency and incompleteness.
For definitions of the different kinds of poetic feet, see the link below.