Read the bugs.
"Match the hatch" is the heart of fly fishing: trout eat whatever insect is active, so if you know which bug is hatching, you know which fly to tie on. It sounds like a lifetime of study - it's really just five groups. Learn these and you'll pick the right fly far more often than you guess.
Nymph → emerger → dun → spinner
WhenSpring and autumn, often mid-morning to afternoon on cool, grey days.
Match withPheasant Tail nymph subsurface; Blue-Winged Olive or an Adams up top.
Larva → pupa → adult
WhenLate spring through summer, and famously on warm evenings.
Match withGold-Ribbed Hare's Ear or a soft hackle rising; Elk Hair Caddis on top.
Nymph → adult (crawls out to hatch)
WhenEarly spring, in faster, rockier, well-oxygenated water.
Match withA big weighted nymph deep; a stimulator or big dry when adults are about.
Larva → pupa → adult
WhenAll year - and often the ONLY thing hatching in winter and on tailwaters.
Match withZebra Midge or RS2 deep; Griffith's Gnat for a cluster on the surface.
Hoppers, ants, beetles - land insects that fall in
WhenHigh summer, warm and breezy, tight to grassy and tree-lined banks.
Match withA foam hopper, a beetle or a sunk ant - fished close to the bank.
Rough hatch calendar
Very rough, typical windows - hatches shift by weeks with latitude, altitude and the weather on the day. Watch the water: rising fish and bugs in the air beat any calendar.
Now put it to work: pick the pattern in the fly box, tie your own in fly tying, and match it to the right rod with the fly rod matcher.
⚠️ A simplified starting point - real entomology is vast and hatches are intensely local. Use this to learn the groups, then let what's actually on your water, on the day, have the final say.