Does Shakespeare exploits constraints in his sonnets?how does shakespeares' sonnets differs from Edmund Spencer's ones?
It might be argued that every early writer of English sonnets--e.g., Howard Earl of Surrey, Wyatt, Spenser--exploited the constraints of sonnets, including Shakespeare. Sonnets were devised by Italian poet Petrarch and followed a set form. Sonnets later became popular in England, but the English language does not lend itself completely to the "constraints" of Petrarchan sonnet structure. So--English sonneteers "exploited" the Petrarchan sonnet to accommodate English and, as in Spenser's and Shakespeare's cases, to allow for communicating either (1) more complex or (2) more connected (either one by adapting the usage of the volta(s)) messages than Petrarch communicated with his single volta. So, yes, Shakespeare did follow the English tradition and exploit the constraints of the sonnet for his innovative purposes.
In his Sonnet 18, Shakespeare follows the English sonnet form; however, rather than have his couplet act as a summation, Shakepeare uses it as an introduction to a new idea, instead. In Sonnet 30, the final couplet again is not a summation; rather, it is a contrasting idea as the persona turns to his new love, and finds a solution to his complaints: "All losses are restor'd and sorrows end." And, in a third example, Sonnet 130 differs in form as the final couplet becomes a contrasting idea rather than a summation as is typical of the Shakespearean sonnet.
I suppose it is easy to consider that a poetic form such as a sonnet is actually a very constrained medium for expressing profound feelings, thoughts and ideas. Actually, if we look at the sonnets, we can see the way that this is completely incorrect. Shakespeare uses (or subverts) the sonnet form as an excellent medium for expressing incredibly deep ideas about love, death and mortality. You might like to pick one of his sonnets and explore how he achieves this.
If I understand you right, you are asking how Shakespeare uses the constraints of the sonnet. I am not sure exploits is the right word, but it seems to me that what we refer to as the Shakespearean sonnet. the more traditional sonnet, fit Shakespeare's needs more than Spencer's versionl. Spencer's fit his needs.