I have just read the sentence like this: " I haven't seen her since 2003. She moved to Australia with her family then". My question is: Why don't we use "she has moved to Australia then"? I have learnt that the present perfect is used for indefinite time in the past and in this sentence we don't know the definite time.
Thanks in advance.
The present perfect is used to speak about the current state. You can't use the present perfect with a time expression that excludes the present.
She has moved to Australia (and so she is there now).
The adverb "then" is giving the time when an event happened. If you are speaking about a past time, you must use a past tense:
She moved to Australia [then = in 2003].
