I'm having trouble explaining the sentence below to my student. He thinks the definite article has something to do with his rationale for using "for" instead of "since".
Skateboarding can be dangerous. Since the 1960s, thousands of people have gone to hospital emergency rooms because of injuries.
I understand the answer in the sentence above is "since". Considering the constructions below however, he asked why "for" can't also be used.
I have lived in Austin since last year. I have lived in Austin for the last year.
Can someone help?