1To strike something against another thing, or to make something hit its intended target.
ボールを壁に当てて遊んでいた。
I was playing by bouncing a ball off the wall.
矢を的の真ん中に当てるのは簡単ではない。
Hitting the dead center of the target with an arrow isn't easy.
彼はサーブでボールをラインぎりぎりに当てた。
He served the ball landing right on the line.
2To expose something directly to light, wind, heat, or the like.
布団を外に干して日に当てた。
I hung the futon outside and let it get some sun.
髪をドライヤーの風に当てて乾かした。
I dried my hair by blowing it with the hairdryer.
この植物は直射日光に当てすぎると枯れてしまう。
If you expose this plant to too much direct sunlight, it will wither.
3To press or place something firmly against another surface, or to apply something onto it (like a patch or bandage).
熱があるので額に冷たいタオルを当てた。
Since I had a fever, I put a cold towel on my forehead.
耳に手を当てて、遠くの音を聞こうとした。
I cupped my hand to my ear and tried to listen to a distant sound.
破れたズボンに当て布を当てて縫った。
I put a patch on the torn pants and sewed it in place.
4To assign a role or task to someone, or (in class) for a teacher to call on a student to answer.
先生は次の問題を私に当てた。
The teacher called on me for the next question.
監督は新人選手をレギュラーに当てることにした。
The coach decided to put the rookie player in the starting lineup.
予算の一部を広告費に当てることになった。
It was decided that part of the budget would be allocated to advertising costs.
5To guess something correctly, such as an answer or an outcome.
彼は私が考えていたことをぴたりと当てた。
He guessed exactly what I was thinking.
テストの山を当てたおかげで高得点が取れた。
Because I correctly guessed which topics would be on the test, I got a high score.
6To score a big win or success in a lottery, bet, or similar chance-based venture.
友人は競馬で大金を当てたらしい。
I heard my friend won a large sum of money at the horse races.
彼は投資でうまく相場を当てて財産を築いた。
He built his fortune by correctly predicting the market with his investments.