I am staying in USA since 20 years.
I am staying in USA from 20 years.
Which one is correct?
I am staying in USA since 20 years.
I am staying in USA from 20 years.
Which one is correct?
In standard English, the form of the verb would not be
I am staying
for something that has been true for 20 years.
One would write
I have stayed
or
I have been staying
One would also write “the USA”, and use “for”, not “since” or “from”. In standard English, your sentence would read:
I have (stayed/been staying) in the USA for 20 years.
You can also consider using remain or live instead of stay.
'Since' is used when the exact duration is not given. e.g. I've been swimming since morning. We are not concerned whether it's afternoon now, or night. That is, exact duration of my swimming is not mentioned.
'For' is used when exact duration is mentioned. e.g. I have been staying in the USA for 20 years. Here also, we are not concerned whether it's 2013 or 2003. Bur here, we are also not concerned what is the starting time period is. I could be staying here since 1983 or 1993, we are not concerned. I've just stated my stay time viz. 20 years.