偶尔 / 不时 / 时常 / 再次: occasionally, now and then, often, once more
【五05】【五06】时间副词、频率副词 / adverbs
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
These four adverbs stand after the subject and before the verb: 偶尔 ("occasionally", rarely), 不时 ("now and then", at short, irregular intervals), 时常 ("often", in narration) and 再次 ("once more", one repetition of a specific event).
偶尔 says a low frequency: it clashes with 每天 or 总是 (✗ 我偶尔每天锻炼). 不时 is not rare: it is something that recurs several times within a short span (老师不时看一下手表). 时常 is close to 常常 (level 2) but more written; in speech people prefer 常常 or 有时候.
再次 is a formal adverb that picks up one specific event (他再次迟到了, 再次感谢). Do not mix it up with 再 (future: 明天再来) or with 又 (a repetition already done, more colloquial: 他又迟到了). It also turns up in written courtesy formulas: 再次向您表示感谢.
Examples
他偶尔去上海出差。
Tā ǒuěr qù Shànghǎi chūchāi.He occasionally travels to Shanghai on business.
我时常想起在大理的日子。
Wǒ shícháng xiǎngqǐ zài Dàlǐ de rìzi.I often think back to my days in Dali.
上课的时候，老师不时看一下手表。
Shàng kè de shíhou, lǎoshī bùshí kàn yíxià shǒubiǎo.During the lecture, the teacher glances at his watch every now and then.
他再次迟到了，经理很不高兴。
Tā zàicì chídào le, jīnglǐ hěn bù gāoxìng.He was late once again, and the manager is very unhappy.
周末我偶尔会去爬山，更多的时候只想在家休息。
Zhōumò wǒ ǒuěr huì qù pá shān, gèng duō de shíhou zhǐ xiǎng zài jiā xiūxi.At weekends I occasionally go hiking, but most of the time I just want to rest at home.
谢谢您的帮助，我想再次向您表示感谢。
Xièxie nín de bāngzhù, wǒ xiǎng zàicì xiàng nín biǎoshì gǎnxiè.Thank you for your help, I would like to express my thanks once again.
Common mistakes
✗ 我偶尔每天锻炼 → ✓ 我偶尔锻炼 / 我每天锻炼 (偶尔 clashes with 每天).
✗ 他不时去美国出差 (once a year) → ✓ 他偶尔去美国出差 (不时 means "repeatedly", not "rarely").
✗ 明天我再次来 → ✓ 明天我再来 (for a plan, use 再; 再次 picks up a specific event).