If and only if used in the same way means the same thing, except that only if is more forceful, more compelling If and only if is the most obligatory of the three, in which the action has been distinguished and emphasised, if, and only if it's the most forceful of the three 2 the oxford english dictionary defines but only (which can also occur as only but) as meaning ‘ (a) only, merely (b) except only’, and comments that its use is now poetical. Only but (also but only) Oxford english dictionary (login required) below are some only but examples from the corpus of contemporary american english
Swap in only or nothing but for only but to see Ultimately, there is only but one choice for you, no To consume the entire pint I can only do so much in this time Or i can do only so much in this time. When only after, only if, only in this way etc
Only after lunch can you play. For only in the upper class, the only restricts in the inclusion (in, as opposed to out) The wording implies that only b matters, not c, d, e, I will help you prepare for the meeting only if you finish your report This implies that finishing the report is a necessary but not necessarily sufficient condition for me to help you prepare for the meeting. In only when , there is a sense of urgency, a slightly more 'involved' writing
It was only when is by comparision more 'relaxed' writing, more like someone is recounting something to someone. However, one and only one adds emphasis to the fact that there is only one, and draws attention to it For example, the student who is the only one who failed, might feel more ashamed if the teacher uses one and only one, as the teacher might be perceived as purposely drawing attention to that fact, for whatever reason.
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