study的过去式和过去分词怎么写

　　Coincidentally, they had both studied in Paris.

　　碰巧的是，他们俩都在巴黎学习过。

　　He studied conscientiously and enthusiastically.

　　他认真积极地学习。

　　Many toxic effects can be studied at the cellular level.

　　许多毒性作用在细胞阶段就可加以研究。