'Do' and 'tial' are often substitutable for each other, but swapping them does subtly change the meaning in most cases.
'Do' is about conclusions, often logical or even just linguistic. (And, by extension, for follow-up questions.)
'Tial' is about causes (including motivations).
So use 'tial' where you're relating two or more events that each have some (often unstated or maybe even unknown) point in time and that make up causal chains.
- I wasn't sure, I'd like the event.
- Therefore, I didn't sign up.
Mi ne certis, ĉu mi ŝatus la eventon.
Tial me ne aliĝis.
Mi ne certis, ĉu mi ŝatus la eventon,
tial me ne aliĝis.
I.e., not being sure caused me to not sign up.
With 'do' we can relate statements, some of which (or whose relation) might exist outside of time.
- I didn't sign up yet.
- Thus [by definition], I'm not a sign-up-ee (=someone who has signed up, a registrant).
Mi ne ankoraŭ aliĝis.
Do mi ne estas aliĝinto.
Mi ne ankoraŭ aliĝis,
do mi ne estas aliĝinto.
I.e., from the fact that I didn't sign up, I can logically (or here even just linguistically) conclude that I'm not someone who has signed up. (Duh.)
Even though me not signing up and me not being a registrant are events or states that both could be assigned to some point in time, their logical relation exists completely outside of time. It's not that I first not signed up, and that then caused me to not be someone who's signed up. Thus, using 'tial' here, while maybe not completely wrong, would at least feel overly heavy and rather strange.
This difference is difficult to grasp, as many languages, including English, while having plenty of different words for these two meanings, use them rather indistinctly for both of these purposes (and some of them, like "so" for even many more meanings than just these two):
- because of that
- for that reason
And even in Esperanto, if you turn the relation around, it becomes less clear what word(s) should be used.
Because I wasn't sure that I'd like the event, I didn't sign up.
Ĉar mi ne certis, ĉu mi ŝatus la eventon, mi ne aliĝis.
As I didn't sign up yet, I'm not a registrant.
Ĉar mi ne ankoraŭ aliĝis, mi ne estas aliĝinto.