En fait/ au fait/ un fait – In fact/ by the way/ a fact
Don’t mix up these expressions as they sound quite similar. They are all based on the very versatile verb faire
En fait – is used to give an explanation or give a clarification. It can be translated as a matter of fact, actually, in fact.
Au fait- introduce a new topic. Its English equivalent is ‘by the way.’
Un fait- is a noun meaning a ‘fact.’
Let’s see some examples in context.
Au fait, qu’est-ce que tu fais pour ton anniversaire ? By the way, what will you do for your birthday?
Je ne sais pas. En fait, je ne veux pas le fêter. I don’t know. In fact, I don’t want to celeberate it.
Un fait certain, c’est que je te donnerai un cadeau. A sure fact, is that I will give you a gift.