Recently, a non-native speaker asked me whether they should say "Why is it so cold?" or "Why is is this cold?". While clearly the former is much more common, I struggled to explain why. Cambridge Dictionary gives:
This: as much as shown or to a particular degree
So: very, extremely, or to such a degree
which doesn't help much to clear things up. The problem is that in the person's native language, both are expressed using the same word. Come to think of it, most languages I know of do not seem to distinguish between them.
How should I explain the difference in usage?