Combining have been and were in this way is perfectly acceptable here, Umed. Often, in language use, deciding whether to use the present perfect or the past simple depends on your perspective on ...

We can use both present perfect and past simple for things that happened in the past. But if there's a result in the present, we tend to use the present perfect. I've lost my phone.

Practice with simple constructions: "Students study grammar," "Teachers explain concepts," "Practice improves skills." Once this feels natural, gradually add modifiers and clauses.