常常 vs 往往 (often vs tends to)

Rainy Mondays

• You notice a pattern: the subway is always more crowded when it rains.
• to rain下雨xià yǔ(modifier / structural particle)的dewhen时候shí hou，underground railway地铁dì tiětend to往往wǎng wǎng很挤hěn jǐ。 — "When it rains, the subway tends to be crowded."

often常常cháng cháng = often (a frequency)

• often常常cháng cháng says something happens often: I我wǒoften常常cháng chánggo to去qùthe library图书馆tú shū guǎn. It can describe the future: after以后yǐ hòuI我wǒcan / will会huìoften常常cháng chángto come来lái.
• It needs no special condition.

I我wǒoften常常cháng chánggo to去qùthe library图书馆tú shū guǎn。

tend to往往wǎng wǎng = tends to (under a condition)

• tend to往往wǎng wǎng describes a regular pattern found under certain conditions: at weekends周末zhōu mò(modifier / structural particle)的desupermarkets超市chāo shìtend to往往wǎng wǎngpeople人rénbe many很多hěn duō。 — "Supermarkets tend to be crowded at weekends."
• The condition (time, situation) is usually stated.

at weekends周末zhōu mò的desupermarkets超市chāo shìtend to往往wǎng wǎngpeople人rénbe many很多hěn duō。

Where tend to往往wǎng wǎng does not fit

Side by side

• I我wǒoften常常cháng chángdrink喝hēcoffee咖啡kā fēi — "I often drink coffee" (frequency).
• I我wǒstay up late熬夜áo yè(modifier / structural particle)的dewhen时候shí houtend to往往wǎng wǎngdrink喝hēcoffee咖啡kā fēi — "When I stay up late, I tend to drink coffee" (a pattern under a condition).

I我wǒstay up late熬夜áo yè的dewhen时候shí houtend to往往wǎng wǎngdrink喝hēcoffee咖啡kā fēi。

Recap

Read the full reference: HSK 4 grammar — Comparing similar adverbs.