loin
in French generally indicates distance or the sense of "far away". The most common I hear it used is when someone is saying that one thing is far away from another or asking if one thing is far away from another.
For example:
C'est loin d'ici? (Literal translation: It is far from here?) (Proper translation: Is it far away?)
Let's work through your two phrases.
au loin
au loin
does usually indicate distance in the direction away from the subject or the speaker. I tend to think of it in English as to the far away
, which is a very literal use that helps me get by with such things when I learn languages. That being said, it can better be translated as in the distance
.
In your example:
Au loin on voit un grand pavillon.
The sentences indicates that there is a large pavillion visible in the distance. Without more context, I can't tell whether or not the subject is a group of people or it is being used in the sense of one sees
.
de loin
de loin
can have two meanings. The first indicates distance in the direction toward the subject or speaker. I tend to think of it in English as from the far away
but again, that's a very literal translation that serves my method of thinking. It is better translated as from a distance
.
For example:
De loin, il semble beau. (Translation: From a distance, he seems beautiful.)
The second meaning of de loin
: This does not have to do with distance in relation to a speaker, but is more expressive as an adverb in the sense of vraiment
. It can be translated as by far
or by a long way
.
For example:
Je suis de plus le plus intelligent. (Translation: I am by far the most intelligent.)
There is a word missing in your sentence, but I tend to think your example:
De loin n'entend pas ce qu'il dit.
is probably meant to be written like this:
De loin, on n'entend pas ce qu'il dit.
Which would roughly mean that from a distance, what he is saying is not heard or literally translated:
From far away, one does not hear what he says.
Stylistically translated:
His words can't be heard from a distance.
So, it seems to me that your example involving de loin
uses the first meaning I mentioned.
You can find more on the word here along with other usages.