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。