"Il fera moins dix degrés."
Translation:It will be minus ten degrees.
How do you tell the difference between "It will be minus 10 degrees" and "it will be less than 10 degrees" ? Just the "de" between moins and dix?
Exactly. Il fera moins de dix degrés. = It will be less than ten degrees.
Would "It will be ten degrees below zero" not be acceptable? That sounds more natural to an American speaker than "minus ten degrees".
Sera vs aura vs fera ? How to differentiate .. I thought of future of suis,avoir & fait as above .. is that correct ?
Yes, fera is the simple future tense.
present → future
il fait → il fera
il est → il sera
il a → il aura
Note here that "il" is the impersonal "it" since it is talking about the weather.
Ideally, use "il va faire" for "it is going" and "il fera" for "it will be" when talking about weather.
In US dialect, we could say, "It will be ten below" (the zero is understood.)