What he would do is

他的做法是
he'd put food powder in the dog's mouth to generate saliva.

给狗喂食让狗分泌唾液
But Pavlov observed that

他注意到
when somebody entered the room

当给它喂食的人
who typically gave him the food powder,

进屋时
the dog--the food powder saliva would start to come out.

狗便开始分泌唾液
And later on if you—

稍后
right before or right during you give the dog some food

在喂食前或者喂食过程中
you--you ping a bell

你摇铃
the bell will cause the saliva to come forth.

铃声就会加速唾液的分泌
And, in fact, this is the apparatus that he used for his research.

这是他在研究时所使用的仪器
He developed the theory of classical conditioning

他通过区分两种条件作用