How do we know when to use 'te' and when to use 'sinä' ?
"Te" is plural and "sinä" is singular.
In British English this would be 'quickly' rather than 'fast' since it's an adverb as above
Does nopeasti mean only fast or can it also mean quick?
It means quickly or rapidly.
If you wanted to use fast as an adjective like "Usain Bolt is FAST" that would be "Usain Bolt on NOPEA"
The "-sti" ending means "in a - way".
It's an adverb.