"From then on" or "since then"?

Do these two expressions mean the same or are they used in different contexts? I wrote "Since then" in an essay for my English teacher but she wrote me "from then on" instead. I wanted to say that two children have lived alone since the moment when their parents died, so I wrote

Since then, the two children lived alone...

And my teacher corrected it to

From then on, the two children lived alone...

Thanks in advance!

Solution 1:

I'm afraid I'm going to have to disagree with LightMikE.

The difference between "since then" and "from then on" is one of grammatical aspect.

"Since then" is spoken about things that are still going on. So it would be correct to write

Since then, the two children have lived alone [implying and today they still do].

But the sentence you have, since it's about something that happened in the past but is no longer relevant, requires "from then on."

Similarly, "From then on" is spoken about things that are no longer going on. So it's correct to write

From then on, the two children lived alone [implying but today they don't].

but it would be incorrect to write

From then on, the two children have lived alone.

Since I don't know whether the two children no longer live alone because they live with other people or because they're dead, it might be clearer to show first-person examples. So these would be correct:

When I was a kid, I ate some disgusting French fries. Since then I've hated French fries, which is why I don't ever order them at restaurants.

When I was a kid, I ate some disgusting French fries. From then on I hated French fries, until my mother made me try some a few years later, and now I love them again.

But to switch "since then" and "from then on" in the above sentences would make them incorrect.