In this example, the speaker will first go to the gym. After that is finished, they will visit the post office. Here are more examples:
Therefore
“Then” is frequently used as a casual way to say “therefore” or “in that case.”






At the end of a statement
It is also used in this way to show frustration or annoyance at someone else's action or decision.
Suggestions
The set phrase "Well then ~" is common as an informal way of making a suggestion.
Than (conjunction, preposition)
Making comparisons

”Than” is basically used when comparing things or situations.
In the last example, someone's arrival is compared to when the speaker thought they would arrive.
Expressing contrast
This usage is similar in meaning to “instead of.”
At the beginning of a sentence, it is used together with "rather."
Exceptions
Used with “other,” it means "except for" and expresses that something or someone is not included.
Here, the laptop is not included in "anything."
Common expressions with “then” and “there”

Finally, let's look at some expressions that use these two words.
Then and there
Meaning: at that specific moment
Better late than never
Meaning: something that happens late is better than something that never happens
Better safe than sorry
Meaning: preparation is important in order to avoid trouble or inconvenience in the future