begin has 9 distinct synonyms spread across 6 of its 11 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its one recorded opposite is end.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
set in motion, cause to start
“The U.S. started a war in the Middle East”
begin to speak or say
“`Now listen, friends', he began”
have a beginning, of a temporal event
“WW II began in 1939 when Hitler marched into Poland”
achieve or accomplish in the least degree, usually used in the negative
“This economic measure doesn't even begin to deal with the problem of inflation”
Israeli statesman (born in Russia) who (as prime minister of Israel) negotiated a peace treaty with Anwar Sadat (then the president of Egypt) (1913-1992)
2 of the 11 recorded senses carry no synonym, antonym or related term, so they are not shown above. Every sense, with its definition and examples, is on the full entry for begin.
Synonyms, antonyms and the broader and narrower relations for begin come from Princeton University’s WordNet. Full sources and licences.