Can I mix in a same sentence present perfect and past simple?
Like: "I have just seen a man, he was fixing his car"
Can I mix in a same sentence present perfect and past simple?
Like: "I have just seen a man, he was fixing his car"
First, your example is fine.
Second, it does not mix present perfect and simple past. It mixes present perfect and past continuous.
Third, only in conditional sentences are there rules for sequence of tenses in different clauses.
Otherwise, you choose tenses to express the facts.
You saw the man in the past. When in the past? Recently. Present perfect is the correct tense to describe the recent past, the almost present.
The man had not completed repairs to his car, but he had started on the repairs. That calls for a continuous tense. And the process went on in the past. Past continuous is the tense that indicates ongoing action in the past.
I suggest that you concentrate on the meaning of the various tenses rather than looking for mechanical rules for applying them regardless of context.