English words pair in fixed ways called collocations — you can't just swap them. We say 'strong wind' but which word goes with rain? 'We got caught in ___ rain.'

"Heavy rain" is the fixed, idiomatic pairing English speakers use for intense rainfall, even though "strong" is the natural adjective for wind — these pairings, called collocations, aren't governed by logic but by convention built up over long usage. Native speakers absorb them naturally, but learners often have to memorize them individually.

"Strong rain" isn't idiomatic English, even though "strong wind" is perfectly natural, illustrating exactly why collocations resist simple substitution rules. "Big rain" suffers the same problem, sounding foreign rather than natural.

Collocation mismatches are one of the most reliable ways to spot non-native English even when grammar and vocabulary are otherwise flawless, which is why native-sounding fluency depends heavily on memorizing these fixed pairs.