She has an English conversation only when she is with her friends. => Only when she is with her friends does she have an English conversation. (Note the inversion "does she have")
I only realized it then. => Only then did I realize it. (Note the inversion "did I realize")
Inversions (Using other adverbs/adverbial phrases):
The film had never before laid on such a sumptuous celebration. ⇒ Never before had the film laid on such a sumptuous celebration.
They had seldom participated in such a fascinating ceremony. ⇒ Seldom had they participated in such a fascinating ceremony.
They little suspected that the musical was going to be a runaway success. ⇒ Little did they suspect that the musical was going to be a runaway success.
Conditionals:
If you need some more money, take an extra 200. (This is a Type 0 or Type 1 conditional. Since it is a general advice, I will rewrite it as Type 1) => Should you need some more money, take an extra 200. (Using "should" makes it more formal) Or: If you should need more money, take an extra 200.
If I were your father, I would insist you practice harder. (This is a Type 2 conditional - unreal present) => Were I your father, I would insist you practice harder. (Inversion of "were" and the subject)
If I had thought of the right words, I would have told them what I was thinking! (You provided #4 again. Assuming this is meant to be a different sentence, this is a Type 3 conditional - unreal past) => Had I thought of the right words, I would have told them what I was thinking! (Inversion of "had" and the subject)