It is/was ~ that ~ 的句型，表示强调、加强语气。把要强调的部分放在be动词后面，再将句子其余的部分放在that的后面。

It ＋ be动词 ＋ 强调的部分 ＋ that ＋ 句子其余的部分

I met Robert and his girlfriend in the park yesterday afternoon.

昨天下午我在公园碰到Robert和他女朋友。

It was I that met Robert and his girlfriend in the park yesterday afternoon.

昨天下午是我在公园碰到Robert和他女朋友。(加强主词)

It was Robert and his girlfriend that I met in the park yesterday afternoon.

昨天下午我在公园碰到的是Robert和他女朋友。(加强受词)

It was in the park that I met Robert and his girlfriend yesterday afternoon.

昨天下午我是在公园碰到Robert和他女朋友。(加强地方副词)

It was yesterday afternoon that I met Robert and his girlfriend in the park.

我在公园碰到Robert和他女朋友是在昨天下午。(加强时间副词)

Wendy bought a sandwich at the convenience store.

Wendy在便利商店买了一个三明治。

It was Wendy that bought a sandwich at the convenience store. (加强主词)

It was a sandwich that Wendy bought at the convenience store. (加强受词)

It was at the convenience store that Wendy bought a sandwich. (加强地方副词)

It was a diamond ring that John bought for Mary.

John买给Mary的是一只钻戒。

It was at a party that I first met him.

我第一次遇见他的地方是在一个舞会里。