Why Change Total Derivative to Partial in dU=TdS-PdV?

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for the equation:
dU=TdS-PdV
why can you can the total derivative to a partial derivative, which makes the equation become
T=(partial derivative U)/(particle derivative S) ?
 
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You are taking the partial derivative of U with respect to S with V held fixed. So you take dV=0, and then divide through by dS.
 
Because dU is an integrable differential one-form.

Daniel.
 
thank you very much! :)
 

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