Question

present perfect or simple past,, I ( KNOW) them for many years.He (HURT) his arm yesterday

Answer 1

I have known them for years. He hurt his arm yesterday.

present perfect se foloseste cand este o actiune continua din trecut care se continua in prezent(ii stiu de ani de zile)
simple past se foloseste cand este o actiune delimitata in trecut(ieri mi s-a intamplat ceva)

Answer 2

Present Perfect: I have known them for many years.
Present Perfect exprima o actiune de durata inceputa in trecut si continuata pana in prezent (inceputul actiunii e marcat cu SINCE iar durata actiunii cu FOR)

Simple Past: He hurt his arm yesterday.
Simple Past exprima o actiune inceputa si terminata in trecut. Pot fi precizate perioadele de timp cand s-a desfasurat actiunea: yesterday, last week/month/year, a week/month/year ago,