It depends on what “le” is.

Before a noun, “le” is an article, you can contract to “du”.
Before a verb, “le” is a pronoun, you can’t contrat : it’s “de le”.

Example :

Je voulais remplir la gamelle du chien, mais j’ai oublié de le faire.

Translated literally: I wanted to fill the (article) dog’s dish, but I forgot to do it (pronoun).

Yes, when “Le”, “la”, or “Les” is before a noun (Object, Place, Thing, etc.), it is an Article. However, when it is before a verb you can not contract and it is a pronoun or object of the verb.

examples:

• Je les deteste.
• Je l’aime.
• Je le ferai.