Relative Pronouns

The van, which was stopped earlier today by police, turned out to be driven by the men suspected of last week's bank robbery

The train which travels directly to Liverpool has been cancelled.  'That' would be incorrect because it would be acting as the subject of the relative clause

The classmates with whom I studied German decided to give our teacher some flowers

That is the factory in which my Dad used to work

The teacher who taught me German will be retiring this year

Gordon, whose nephew you met yesterday, will be visiting later

There's the book that I thought I'd lost - 'which' would also be correct

The man with whom the Inspector was speaking is the notorious diamond thief, Renard.  - When a person is the object of a sentence, the correct pronoun to use is 'whom' (The Inspector is speaking to Renard - With whom is the Inspector speaking?).  Because this is considered very formal, you will rarely hear it in spoken English (usually, people use 'who' instead).  It is important to know when to use 'whom', however, for times when you need to be absolutely correct

The woman who said hello was my old dance teacher. Although it can be used to refer to a person, 'that' is not correct as the subject of a clause (i.e. that said hello). You will hear 'that' used incorrectly in informal spoken English, however

Who took the laptop which I was working on? - Many people (including teachers) believe that it is incorrect to end on a preposition ('on') in English.  This is not at all true.  English is a Germanic language - the 'not ending on a preposition rule' only applies to Romance languages (those based on Latin).  'Who took the laptop on which I was working' would sound silly because the grammar is too formal for the context