à + le  au

à + les  aux

de + le  du

de + les  des

There is no contraction with la or l'.

The construction with de is called the partitive because it refers to part of a whole. In English, we often omit the words some or any. In French, you must use the partitive whenever the sense of the sentence limits the quantity to which you are referring. Ask yourself "Do I mean all of the concept referred to or only part of it""

J'achète du lait - I'm buying some milk.

Il commande de la viande - he orders some meat.

Demand de l'eau, s'il te plait - ask for some water, please.

After most negative constructions, the indefinite articles un, une, des become de (d').

Il achète du vin - Il n'achète pas de vin.

Je mange de la viande - Je ne...