if only I knew her name

English as a Second Language English Vocabulary, Grammar and Idioms
1
  1. I can tell her name if only I knew her name.
  2. I can give a lift if only I had gone by taxi.
  3. Only if a teacher has given permission is a student allowed to leave the room.
  4. Only if a teacher has given permission a student is allowed to leave the room.
  5. Only if my mother has given permission I can come to cinema.
    Please correct the above. Thanks.
2

This is how I would modify your sentences, though you may not have meant so in all the cases:

  1. I can tell her name only if I know it.
  2. I can give you a lift only if I go by taxi.
  3. Only with the teacher’s permission is a student allowed to leave the classroom.
  4. A student is allowed to leave the classroom only if the teacher has given them (?) permission.
  5. Only with my mother’s permission can I go to the cinema.

If only is different from Only if.

If only I knew = I wish I knew = I do not know though I liked/wanted to know.

Watch out for other comments.

3
  1. I could tell you her name if I knew it.
  2. I could have given you a lift if I had gone by taxi.
  3. Only if a teacher has given permission is a student allowed to leave the room.
  4. A student is allowed to leave the room only if a teacher has given permission.
  5. Only if my mother has given permission can I come to the cinema.
4

Beeesneees,
Could you please coin a sentence using ‘If only’?
Thanks.

5

‘If only’ is used to say that you wish something was true or that something had happened. Recasting your original sentence, you could say this:
I could write to her if only I knew her name. / If only I knew her name, then I could write to her.
If only I were rich then I could pay for the operation.
I could pay for the operation if only I were rich.
If only he’d remembered to send that letter then he’d have all the information he needed now.
He’d have all the information he needed now, if only he’d remembered to send that letter.
If only I had done my homework, I would not have found my work so difficult now.

You can see that it’s possible to leave ‘only’ out and use ‘if’, but ‘only’ serves to emphasise the longing and desire that this was the case.

Most of the time ‘if only’ is used in context with an unstated reason, so in a conversation where it is obvious that the speaker wishes to write a letter to the girl whose name he doesn’t know, he might just say, “If only I knew her name,” and not add any more information to that. It would be unnecessary because the listener understood what he was talking about.
Similarly, within a context where the listener/reader understands what has happened and what the speaker/writer wishes:
If only I were rich.
If only I’d remembered to send that letter.
If only I’d done my homework.