then

then is on the Basic English 850 List.

the - then - theory

then is one of the 1000 most common headwords.

the - then - there

Preposition

(At) that time or that moment.
- Antonym: now
Then she was little. Now she is big.

The then president did a lot of developmental activities.

We will arrive before then.

I'm going to see you next month; by then you will have finished your thesis.

Your life since then must have seemed very ordinary.
After something else.
First, I cook the food; then I eat it, and then I wash the plates.
In that case.
If she is happy, then I'm happy too.