WHICH/THAT and WHO/M ... 01   see also WHICH 04

'qui' is generally used for 'who' or 'which' when it denotes the *subject* of a following verb.

The man who sweeps the street is tired.	L'homme qui balaie la rue est fatigué.
The door which is open is blue.	La porte qui est ouverte est bleue.

'que' is generally used for 'whom' or 'which'/'that' when it denotes the *object* of a following verb.

I had recognised the man whom I had seen at the market.	J'avais reconnu l'homme que j'avais vu au marché.
I have spent all the money that you have given me.	J'ai dépensé tout l'argent que vous m'avez donné.

However 'qui' is also used for 'who/m' (ie a person) when it follows a preposition (even if it is not the *subject* of a following verb).

I have written to the girl with whom I worked.

J'ai écrit à la fille avec qui j'ai travaillé.