Use than for comparisons.

Use then for time, sequence, or result.

In other words:

So:

That is the core difference.

✅ This test is harder than the last one.

Here, than compares one thing with another.

✅ We checked the schedule, then we booked the tickets.

Here, then shows sequence: one thing happened after another.

✅ If that’s true, then we need a new plan.

In this case, then means something like in that case or therefore.

✅ He is smarter than I expected.
✅ I called him, and then he replied.

The first sentence compares. The second shows what happened next.

The most common mistake is using then when the sentence is making a comparison.

For example:

❌ This movie is better then the first one.
✅ This movie is better than the first one.

Why? Because better sets up a comparison, so you need than.

Another common mistake is forgetting that then does more than just indicate time. It can also introduce a consequence:

So a simple way to avoid confusion is to ask:

Am I comparing, or am I moving the sentence forward in time or logic?

Use this memory rule:

A simple shortcut:

If the sentence contains words like:

you probably need than.

If the sentence means:

you probably need then.

Or even shorter:

comparisons use than
everything-next uses then