<p>What distinguishes </p>
<p>"Had I --------"
"If I were----"</p>
<p><em>Had I been at the scene of the accident, I could have administered first aid to the victims.</em> </p>
<p>How would that same sentence be expressed in the form of " If I were" ?</p>
<p>The “had I” implies an actual situation in the past, the “If I were” implies a possible situation. </p>
<p>Assuming you cast the phrases in the same sense, there is no real distinction. The equivalent you asked for is “If I had been . . .” Beginning with the auxiliary accomplishes the same thing as beginning with “if.” </p>
<p>“Had I…” precedes a verb.
“If I were…” precedes a noun or adjective etc. </p>
<p>Had I presented myself… (present is verb)
If I were present (present is adjective. Were is the verb here.)</p>
<p>They are just two ways of saying the same thing. </p>
<p>Edit: If you started that sentence with “If I were” it would be: “If I were at the scene…”</p>