Use the present perfect with for and since to talk about something that started in the past and is STILL true now. Use for with a period of time, and since with a point in time.
Quick tip: for + a period (for 5 years). since + a point in time (since 2019). Still true now → present perfect.
FOR + a period of time
Use for with how LONG: for two hours, for a week, for five years.
I've lived here for five years.
She's worked there for six months.
We've known each other for a long time.
They've been married for ten years.
SINCE + a point in time
Use since with WHEN it started: since Monday, since 2019, since I was a child.
I've lived here since 2019.
She's worked there since January.
We've known each other since school.
He's been here since this morning.
Still true now (use present perfect)
The action started in the past and continues now, so use have/has + past participle — not the past simple.
I've studied English for two years. (I still study it.)
Camila has travelled since she was young.
We've stayed at this hotel since Friday.
NOT: I studied English for two years (= I stopped).
Common mistakes
Not "I've lived here since five years." — say "I've lived here for five years.". five years is a period → use 'for'.
Not "She's worked there for January." — say "She's worked there since January.". January is a point in time → use 'since'.
Not "I live here for three years." — say "I've lived here for three years.". Started in the past, still true now → present perfect.
Practise it
Is the time a period (an amount) or a point (a start)? Period → for. Point → since. And it's still true now, so use the present perfect.