when I usually use these words?magnificent , phenomenal , terrific ,splendid...

onelook.com might be good place to look

There is not much difference between those words in common speech, they just mean "good/exciting/significant". By the way, you missed out "awesome" and "cool".

The only thing I can think of that could distinguish these words is that some don't always have a positive connotation. Terrific could mean monstrous. Tremendous could mean formidable. Wonderful, however, wouldn't be used in those senses.

Antonyms of these words could be unremarkable, ordinary, unimpressive, dull, unexceptional, unspectacular, and insignificant.

thanks guys.