つつ

1. 1Simultaneous action: the subject does one thing while doing another.Ex. 1
2. 2Concession: the subject acts despite knowing or thinking something that would point the other way.Ex. 2

つつ is mainly a written or formal pattern, common in essays, news, narratives and speeches. It means much the same as ながら, but ながら is more neutral and is the usual choice in everyday conversation. The concessive sense often appears in set phrases such as 知りつつ and 思いつつ, where the subject acts despite what they know or think; this may carry regret, but it can also describe a deliberate choice.

• つつあるN2「つつある」also attaches to the verb continuative form and belongs to written language, but it indicates a change that is gradually progressing and not yet complete; it attaches only to verbs of change and does not link two actions, so it cannot be interchanged with 「つつ」.
• ながら / いながら / ながらもN3「ながら」also carries both the 'while doing...' and concessive meanings, but its register is neutral, and it is the usual choice for simultaneous actions in everyday conversation; in its concessive use it can also attach to adjectives, nouns, or 「ていながら」, whereas 「つつ」attaches only to the verb continuative form.
• 〜つつもN3「つつも」expresses only the concessive meaning and can replace 「つつ」in that use, with 「も」making the contrast clearer; for the 'while doing...' meaning only 「つつ」is possible, and it cannot be replaced with 「つつも」. Both attach to the verb continuative form and both belong to written language.
• つ〜つN2「つ〜つ」requires a pair of verbs (usually antonyms or an active/passive pair) and expresses actions alternating repeatedly, as in 「浮きつ沈みつ」, and is essentially limited to fixed expressions. 「つつ」attaches to a single verb and expresses simultaneity or concession, so the two cannot be interchanged.
• て / でN5「て／で」expresses cause, with the following clause describing the result or feeling that arises from it, and its register is neutral. 「つつ」expresses simultaneous action or the concessive 'although... yet...' meaning, belongs to written language, and attaches only to the verb continuative form; their meanings differ, so they cannot be interchanged.

つつ means "while" or "although", unlike つつある, which describes an ongoing change ("is in the process of"), and つ〜つ, which pairs two verbs for back-and-forth actions (持ちつ持たれつ). ながら has similar meanings but is more neutral and common in speech, and つつも marks the concessive sense explicitly. て/で, by contrast, mainly links actions in sequence, shows means, or describes the manner of an action (手をつないで歩く), and it lacks つつ's formal written tone.