When you're not sure, you speculate instead of stating a fact. May, might, and could all point to something possible rather than certain, but they don't feel identical. May means "there's a real chance" and is the most formal or careful of the three — common in notices and polite speech. Might also means "perhaps", but sounds a little less certain and more everyday. Could means "this is one possibility" — it often floats one option among several. So they overlap in meaning, yet may is the most formal, might the most tentative, and could the most "here's an idea".
Guessing with might, could, may
might /maɪt/ — Amina might join us at the library after work.
could /kʊd/ — The delay could affect the evening performance.
may /meɪ/ — The clinic may close early during the storm.
Not "Chatting with a friend: "It may rain, so maybe bring a coat." (may sounds stiff here)" — say "Chatting with a friend: "It might rain, so maybe bring a coat." (might sounds natural)". In everyday talk, might and could sound natural; may is more formal — save it for notices or careful, polite speech. They aren't freely swappable.
Try it: Sofia: Do you know where Mei is? Which reply from Lani is the careful guess? (Mei could be in the reading room, but I haven't checked. / Mei is in the reading room. / Mei was in the reading room yesterday.) Answer: Mei could be in the reading room, but I haven't checked. — 'Could be' marks it as a guess, which fits because you have not checked. 'Mei is' states it as a fact you do not actually have, and the past-tense option answers a different question (where she was yesterday).
Imagining a situation: what if, suppose
To raise a hypothetical — a "what would happen if…" situation — open with What if …? or Suppose …. What if …? means "imagine that this happens" and is common in everyday, spoken English, often to float a worry or a snag. Suppose … means "let's assume for a moment that…" and feels a touch more thoughtful and formal. Both invite the other person to imagine a possibility with you, rather than describe something real.
what if /wɑt ɪf/ — What if the buses stop running before the concert ends?
suppose /səˈpoʊz/ — Suppose the room is full, where should we meet?
Not ""If you can't find the email?" (sounds like an unfinished real condition)" — say ""What if you can't find the email?" (clearly opens an imagined problem)". What if and suppose flag that you're imagining a situation, not reporting a real one.
Try it: Sofia: The booking is all sorted, I think. Diego: ___ you can't find the booking email? Which reply raises an imagined problem? (What if / Because / When) Answer: What if — 'What if' opens a hypothetical, asking the group to imagine a possible problem. 'Because' gives a reason for something real, and 'When' treats it as certain to happen, not as a guess.