Sometimes means on some occasions and not on others. Sometimes he walks to work, sometimes he takes the bus. Sometimes the lift is broken and you find out on the sixth floor.
It sits comfortably in three places in a sentence — at the front (Sometimes I forget), in the middle before the main verb (I sometimes forget), or at the end (I forget, sometimes) — and the front position gives it the most weight. It sits between rarely and most of the time, and it refuses to say which end it is nearer. Keep it apart from sometime, one word and no -s, which means at a time not yet fixed: come round sometime. Keep both apart from some time, two words, which means a fairly long while: it took some time to fix. Sometimes answers the question how many times; the other two answer when and how long.
1adverbsometimesOn some occasions but not on all of them.
“Sometimes the post arrives before eight, sometimes not until noon.”
synonyms: occasionally, now and then, at times
Also accepted in games: occasionally
Middle English wrote it sumtymes, which is sometime with an -s stuck on the end. That -s was the old English way of building an adverb out of another word, and it left its mark on always, besides and nowadays. Which is why the -s will not come off: sometime and sometimes are two separate words now, not one word with an optional ending.
Middle English
sumtymes
English, today
sometimes
From Middle English sumtymes, somtymes, som tymes, equivalent to sometime + -s. Compare West Frisian somstiden (“sometimes”), Dutch somtijds, somwijlen (“sometimes”).
Etymology from Wiktionary, CC BY-SA