probably necessary
useless>pointless
courteous
necessary
Pointless
beautiful
obsolesce
useless
useless
necessary necessary
necessary handy
The letter s by itself is never an entire syllable - not nes-s-ary. Learn to hear the "c" in the word - ne-ces-sary. The antonym would be unnecessary. Or irrelevant, useless, beside the point, etc.
beginning _____ I don't think so. Antonyms will be the same part of speech. Begin or began would be antonyms for quit, and quitting would be an antonym for beginning.