I thank everybody for the explanations; the question remains, for me, a little complicated. My doubts were originated by some translations, that follow, I found in an on-line dictionary (I translated the Italian translation into English)
она подучила его немецкому языку — gli insegnò un po' di tedesco (he taught him a little German); in this case подучить is translated as learning a little, only the basics of German.
Её подучили, она стала медицинской сестрой" (В. Панова) — "Imparò a fare l'infermiera" (V. Panova) (she learned how to work as a nurse); in this case, not knowing all the Russian text, in my opinion the translations could be the following two:
a. somebody taught her the basics in order to allow her to work as a nurse, may be in order to face an emergency situation;
b. she had acquired before a professional experience as a nurse and this experience has been improved by adding something;
- перед отъездом в Германию он подучился немецкому — prima di andare in Germania imparò un po' di tedesco (before leaving to Germany he learned some German); in this case, as the first, подучить has been translated as learning a little.
I thank for every clarification.