I learnt that so, as, and also have overlapping etymologies. Also and as come from the same Old English word, but are used differently today.

Modern EnglishOld English

The places where you can use so, as, and also overlap and diverge in confounding ways. Take one example, a construction which survives from Old English equating two things linked by so:

So does a father, so does his son

Originally, eallswā was an intensified version of swā. I think of it as, "the same in all ways, not just some". Indeed, you can interchange so and as in this sentence and the meaning is the same:

As does a father, so does his son

Let's try that with also:

*Also does a father, so does his son

Hmm... that doesn't sound right. But if we just swap it with a different so, it works (and this is the order that is more common in Old English):

So does a father, also does his son

But this order doesn't feel right when we use as:

*So does a father, as does his son

You can keep playing around like this. Try two as, or an as and an also, and see if it sounds permissible or not. It's a little mind-bending.

I pity people who learn English as their second language.