Translation:My father-in-law has knitted for several years.
I had never heard "knit" used as any kind of past, but it's attributed in Cambridge. You've taught me something there. Thank you.
"Has knitted" is the present perfect tense, which can be a present tense or a past tense.
In any event, with "since" or "for" + present perfect, the French translation has "depuis" + the present tense to describe the situation as it is now, while "depuis" tells you when this situation started.