"We doubt it."
Translation:Nous en doutons.
That translates to "We doubt of that" Learning to use en is necessary to learning French.
I would slightly expand the answer given by relox84. 'To doubt something' is another way of saying to have a doubt ABOUT something, which is the idea conveyed by 'de'. So as 'en' has the sense of 'de + le/la' putting 'en' into the sentence makes it mean 'doubts about it'. As someone else has said, this use of 'en' works with all verbs which have to be followed by 'de'.
Someone else queried the phrase 'il doute' without de. That would translate in English as 'he has doubts' without stating what the doubts are about (possibly because it is part of a discussion and the matter is understood by everyone involved).
I got an e-mail saying you had responded to my last statement, but I see you were really asking relox84 about his.
"In French, "to doubt something" is "douter de quelque chose", and "de ça" gets replaced by "en""
Apparently you have to understand the "de quelque chose" is understood to go after the verb "douter", and then the "quelque chose" is a "that" (ça) and you can convert "de ça" to "en".
It sounds crazy to me, but that's how I understand what he wrote.
So, at the end "Nous en doutons" means (unscrambling) "Nous doutons de quelque chose" or "Nous doutons de ça".
All those apparently mean the same thing: "We doubt that".
Good luck with your study!
The problem is that prepositions do not translate well from one language to another. We might say that "We doubt about it." but we are more likely to say "We doubt it." in French they use a different preposition and it is important to learn which verbs use which prepositions. https://www.thoughtco.com/french-verbs-with-prepositions-4078811
Please scroll down at the second link for more than one list of verbs that use "de".
From what I've read from others, il doute means "he doubts" or "he has doubts" without specifying what it is he has doubts about. So il doute can stand alone.
Once you start going into what he has doubts about it becomes "il doute de...". So now when the object of his doubts is made into a pronoun to be placed before the verb, it becomes en. As pronouns seem to do when they involve "de".
This is what I gather feom various different threads. Someone please correct me if I'm wrong.