sometime,at times,every now...

個人理解:SOMETIMES=有時(冇讚,冇踩)
AT TIMES係有條件A嘅有時,如果條件A出現得密,AT TIMES可以係時時
NOW AND AGAIN係一而再,再而三,次數唔多,但令人反感.
EVERY NOW AND THEN係強調一路都係咁(時間長度),冇變...
所以用得最多係SOMETIMES.

at times, every now and then, now and again這三個 phrases 意思相同. sometime有另外一個意思.

SOMETIME (adverb) = at a time in the future or in the past, although you do not know exactly when or the time has not yet been decided.
在某一時候; 日後, 改天; 遲早, 總有一天

Let's get together sometime. 有時間我們聚一下吧 (future)
It happened sometime last month. 此事發生在上月的某個時候. (past)

SOMETIME (adjective) = former 以前的

He is my sometime boss. 他是我從前的上司

Do not confuse sometimes with sometime. Sometimes is an adverb of frequency. It means “on some occasions”, “more than once". (past, present or future)
I sometimes went skiing when I lived in Sweden.
我有時會去滑雪,當我住在瑞典時.

AT TIMES = sometimes, but not often 有時，偶爾

He's a real gentleman, though a little pompous at times.
他可是個君子，雖然有時略顯得傲慢了些

EVERY NOW AND THEN = NOW AND AGAIN = sometimes, but not regularly or often 有時，偶爾, 不時地

I write to him every now and then. 我有時寫信給他

Every now and again she went upstairs to see if the baby was still asleep.
她時而到楼上看看嬰孩是否還在睡著


題外:
To 回答者： TOMING88: No offence

Tung Kin Wah, sometime Chairman of this country.

Your phrase may be grammatically correct, but you …………
First, you spelt his name wrong. I guess you meant 董建華 ~ .前香港特首
Chairman of this country 國家主席 ?????

Tung Chee Hwa, the sometime chief executive of the Hong Kong S.A.R. of China

sometime=adv.not know exactly=I saw him sometime last Winter.
adj.what sbd used to be=Tung Kin Wah,sometime Chairman of this country.
at times=when sth happens eg What are you doing at times?
every now&then=each alternate one,every other one eg. We see &meet each other every now&then.
now&again=(every)now&again=from time to time,occasionally;eg(Every)now&again she checked to see if the baby was still sleeping.