The imperative of "distrarre" is "distrai!". This is like "fare", whose imperative is "fa'" or "fai". With an object particle it becomes "fallo". Does that happen with "distrarre" (and "trarre") as well? Because "distrallo" and "trallo" sound terrible to me, but a friend of mine is convinced they're right. I would say "distràilo" and "tràilo". Which is right?
(dis)tràilo and tràilo are correct.
(About the mere words and their existence in Italian it comes to my mind only strallo: a steel cable for buildings or in boats)
But why in tràilo the "i" pre-clitic is preserved while in dallo and fallo is "lost"?
Probably because dare and fare (together with dire, stare, andare) don't follow the "usual" pattern of conjugation and have an apocopated/shortened form.
This phenomenon is known as univerbation with syntactic gemination (or syntactic doubling [raddoppiamento sintattico in Italian]) that occurs when clitics (clitici in italian) are welded with strong monosyllabic verbal forms (typically the second person singular of the imperative).
- fare: imp. fai, fa' → fa'+lo=fallo, fa'+ti=fatti
- dire: imp. di'→ di'+lo=dillo, di'+ci=dicci
- dare: imp. dai, da'→ da'+lo=dallo, da+mi=dimmi
- andare: imp. vai, va'→ va'+ci=vacci
- stare: imp. stai, sta'+ci=stacci
The second person singular, thanks to his affective content (volitive), is inclined to shorten (see Rohlfs, Historische Grammatik der italienischen Sprache und ihrer Mundarten, Vol. 1, §606).
See for example:
- tenere: imp. tieni, tieni+lo=tienilo but we have
Tienlo a mente, e legatelo al dito.,
E tienlo un poco in collo, almen tanto, che pianga la madre.
(cfr. colloquial forms te’ ‹tè› or tie’ ‹ti̯è›)
- venire: imp. vieni, vieni+ci=vienici but we have
Siam derelitti pargoli. Deh! vienci a benedir.
With the verbs trarre and its compounds (contrarre, distrarre, attrarre, etc.) we don't have vowel apocope or shortening of the regular form in the imperative and we have univerbation only:
- trarre: imp. trai → trai+lo=trailo
- distrarre: imp. distrai → distrai+lo=distrailo
- attrarre: imp. attrai → attrai+le=distraile