The poem "Mirror" by Sylvia Plath is written in free verse. This means that it does not follow a fixed metrical pattern, but rather that the rhythm of the lines is meant to project the emotions the poet intends to evoke in the reader.
The poem is separated into two stanzas, each consisting of nine lines. The lines range from eight to fifteen syllables long, but they are not grouped into feet as would be the case in an accentual-syllabic poem and there is no regular system of line lengths as is syllabic poetry. The lines are usually end-stopped, meaning that the ends of lines coincide with syntactic units, usually ends of clauses or sentences, rather than syntactic units being split by line breaks.
The poem mainly consists of short declarative sentences, sometimes with appositive constructions. Plath tends to use simple words of one or two syllables in this poem. The longest and most complex sentence is found in the three final lines of the poem.
This is a poem that is written in free verse, which means there is no rhyme or any particular meter to the poem. The poem is based around two stanzas of nine lines each. The form of the poem is based around the mirror's voice as it describes what it sees and the relationship that it has with the woman that owns it. Of course, to get a good mark, you will need to somehow connect the form of the poem to what Plath is trying to communicate, which can often be where students fall down.
You might like to think about the way in which this is a poem that deliberately helps create the voice of the mirror, which, as it tells us, is "silver and exact." The mirror does not speak in flowery language and is accurate and detached in what it says and how it says it. Therefore, having a form that is not "poetic" in the traditional sense through its use of free verse is something that helps create the impression of the mirror.