〜ごとに
every, each
Noun / Number + counter + ごとに
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ごとに means "every" or "each": オリンピックは四年ごとに開かれる, "the Olympics are held every four years".
With units, it's a regular interval: 一時間ごとに, "every hour". With nouns, it means "each, one by one": 駅ごとに止まる, "stop at every station"; グループごとに発表する, "present group by group".
It also describes change with each unit: 季節ごとに景色が変わる, "the scenery changes with each season"; 一雨ごとに暖かくなる, "it gets warmer with every rainfall".
Compare たびに, for occasions ("every time I travel"), and おきに, which with countable things skips one. Before a noun, ごとに becomes ごとの: 月ごとの売り上げ, "monthly sales".
Examples
オリンピックは四年ごとに開かれる。
The Olympics are held every four years.
駅ごとに止まる電車に乗った。
I took a train that stops at every station.
季節ごとに、町の景色が変わる。
The town looks different each season.
一時間ごとに休憩をとってください。
Please take a break every hour.
グループごとに発表してください。
Please give your presentations group by group.