The police officer (who/that) I spoke to was working on the reception desk / The police officer to whom I spoke was working on the reception desk.

Our computer. which we only bought two months ago, keeps on crashing.

They gave us a present, which was a complete surprise

My girlfriend, who is an architect, is very intelligent / My girlfriend, who is very intelligent, is an architect.

The things (which/that) I left on the table aren't there any more.

I've got two brothers, neither of whom can swim.

It's too hot in my flat, which makes it impossible to sleep.

The car which/that crashed into mine was a Mini.

The houses, many of which were built in 1870, are still in very good condition.

That's the electrician who/that did some work for my mother.