Edmund Spenser's "Sonnet 75" is a Spenserian sonnet, so it has a specific structure/form that it follows, but it is not possible to discuss the form of this sonnet without discussing the rhyme scheme. At first glance, the sonnet appears to be a Shakespearean sonnet. It is made of three quatrains followed by a couplet, and written in iambic pentameter. That's the same form that Shakespeare uses. The similarity is also identical if a reader is looking only at the rhyme scheme of the first quatrain. Shakespearean sonnets use an ABAB rhyme scheme for the first quatrain, and "Sonnet 75" by Spenser does the same thing.
One day I wrote her name upon the strand,
But came the waves and washed it away:
Again I write it with a second hand,
But came the tide, and made my pains his prey.
The key to Spenser's sonnet is found when a reader looks at the rhymes leading the start of the next quatrain. Spenser uses “couplet links” between quatrains. Lines 1-4 will be ABAB, lines 5-8 will be BCBC, and lines 9-12 will be CDCD. What this does is create couplets within/between the stanzas.