"Olen aavikolla" - "I'm at the desert"
"Olen aavikossa" - "I'm in the desert"
So, +lta is for something you're at and +sta for something you're in. For aavikko I'd always say at / +lta. I think in most cases you just have to know.
"Olen aavikolla" - "I'm at the desert"
"Olen aavikossa" - "I'm in the desert"
So, +lta is for something you're at and +sta for something you're in. For aavikko I'd always say at / +lta. I think in most cases you just have to know.