Understanding Only and when it can indicate sufficient

Hi, I wasn't sure where to post this but in studying I came across something that I want to wrap my head around since I've been studying with the approach "only" = necessary, and "the only" is the only exception, but I think I've found something fairly easy to overlook where this is false. I'd like to ask for someone more experienced in this manner for their work around and grasp for the extent and rules for when only can indicate sufficient and if can indicate necessary ???? and whether there are rules governing it. (please let me know if any of this is mistaken.) ie: when it rains it only pours. rain -> pours (conventional) it will only pour when it rains. pour-> rain my interpretation: from the fact that it pouring it is necessary that it rains. (the reverse needed not be true: if rain it can drizzle) but as you may notice, typically "when" indicates sufficient (when I sleep i close the curtains), but here "when" represents the necessary term, and also, only is the sufficient term!?!?! it feels like the style here is x will only happen if y happens (note the only and if). moreover the term only is not irrelevant it will pour when it rains rain -> pour when is now sufficient - i also tried it with "if" it will only pour if it rains. pour -> rain (!?!?!) similarly I tried to understand/reason it in the reverse way but I believe its incorrect - it can rain without pouring - but if it pours it has to rain. moreover a slight switch to it will pour only if it rains pour -> rain then it follows the only if rule, where rain is in fact necessary for pouring. but in making this I neither added nor deleted words, but now only (modifies) to rain whereas the only (I think) previously modifies (pour). either way I'm left super unsure now with the rules of sufficient and necessary. thus, my question is especially since both a term used for sufficiency (when, if) and a term typically indicating necessary (only) are both present in a single statement - where the differences in meaning would literally be moving a word in front or after - how do I make sure to pick up on it, what the condition and exceptions that make it the case here and if there are more general conditions I'm not seeing where the formal rule is moot(for if=sufficient, only=necessary, and does this also apply to the rest of the vocabulary for sufficient and necessary!!) ? - is only the only necessary term where it can be sufficient ? Lastly, along those lines, also how can I pick up on this reverse relationship especially where the logic chains get longer).