What's the difference between 'starý' & 'stàří'? I got the second one right but not sure how it's supposed to be used.
Starý is used with singular masculine INanimate nouns (nominative and accusative), while staří is used with plural masculine animate nouns (nominative).
When is it appropriate to use mašiny instead of stoj?
In this course always. Mašina is quite colloquial though.