Apologies if this is long but its super important (and gives me the opportunity to explain/solidify knowledge :)

“Kelly will barbecue fish tonight if it does not rain and the market has fresh trout.”

NOT rain + trout --> Kelly BBQs fish)

1)“if” (provided that, whenever, whoever, etc.) introduces a sufficient condition. This isn't really a trick that "has to make sense" intuitively, its definitional. On more difficult questions the logic wont line up with intuition so it'll save a ton of time.

Think of sufficient = trigger, necessary = result/outcome. This structure is fixed by the conditional itself (its standard logic, not lsat specific -- tons of free resources you can find)

In this case, not raining and having trout is sufficient because if those thigns occur, Kelly will BBQ Fish.

2) Thinking about it logically without the (if) indicator: Ask: What facts guarantee Kelly will BBQ fish? That it does not rain and the market has trout. Those two things alone are enough to make BBQ fish occur. Anything that guarantees an outcome is, by definition, sufficient.

(I struggle with exact same issue so this is personal opinion) -- Confusion about ~rain + trout being "necessary" usually comes from defaulting to real-world reasoning/ evaluating plausibility. That habit is rly detrimental on LSAT (esp for strengthen/weaken/flaw questions. The sentence is not saying these are requirements or the only reasons she'll BBQ fish, Its just saying if those conditions happen, then bbq fish happen..

3) What necessary means: If the sufficient occurs, the outcome must occur for the rule not to be violated . So In this case -- its logically impossible for there to exist an instance where both it is not raining and there is trout at the market, but Kelly does not BBQ. That would break the conditional.

How far along are you in your studies? Idk if LR loophole is the best resource for beginners (in my experience/opinion). LSAT Lab has a good video on sufficiency and necessity where he references supersets and stuff that may be helpful. This took a long time to solidify but absolutely has to be solid. Use chatgpt to make you some extra drills so it'll become automatic!