顺便 (shùnbiàn) (בעוד שאתה עוסק בזה / אגב)

A library trip can include a second errand

• I / me我wǒto go去qùlibrary图书馆tú shū guǎn，while at it顺便shùn biànstill / also还háithis这zhè本书běn shū。 The trip provides an opportunity to return the book as an additional action.
• while at it顺便shùn biàn presents an action as incidental to another purpose or opportunity. It does not measure the number of minutes or prove the task is easy.

Put while at it顺便shùn biàn before the additional action

• you你nǐgo out出去chū qù，while at it顺便shùn biànbuy买mǎisome点diǎnfood菜cài。 The additional request is buying some groceries while the person is going out.
• The main action can be supplied by earlier context. while at it顺便shùn biànto ask问wèn(used after a verb) give it…一下yī xià introduces a side question without needing a new trip.

you你nǐgo out出去chū qùwhile at it顺便shùn biànbuy买mǎisome点diǎnfood菜cài。

Name the main purpose and the extra

• I / me我wǒto go去qùto see看kànfriend朋友péng yǒu，while at it顺便shùn biàn(把: marks the object)把bǎbook书shūto return sth to sb还给huán gěishe / her她tā。 Visiting the friend is the stated main activity; returning the book is the extra.
• I / me我wǒspecialist专门zhuān ménto go去qùstill / also还háibook书shū presents returning the book as the purpose of a special trip. The difference is the role of the action, not a grammar verdict about the word specialist专门zhuān mén.

A special trip can still have an extra

• I / me我wǒspecialist专门zhuān ménto go去qùbank银行yín hángto handle (affairs)办事bàn shì，while at it顺便shùn biànbuy买mǎi(completed / change)了lebread面包miàn bāo。 A special bank trip can also provide an opportunity to buy bread.
• specialist专门zhuān mén and while at it顺便shùn biàn can coexist when they describe different purposes. Identify what each word modifies before claiming a contradiction.

Do not invent convenience details

• while at it顺便shùn biànbuy买mǎi(completed / change)了lebread面包miàn bāo says buying bread was an additional action. It does not state that the shop was next door, the queue was short or the bread was cheap.
• Those details need their own evidence. An incidental task can still take effort or time.

Use a side question naturally

• while at it顺便shùn biànto ask问wèn(used after a verb) give it…一下yī xià，you你nǐweekend周末zhōu mòto have time (to do sth)有空yǒu kòng(yes/no ?)吗ma？ introduces an additional question in the exchange. It need not describe physical travel.
• For a new example, identify the existing purpose or conversation and the added action. Translate that relationship instead of adding an unproved easy-task claim.

Read the full reference: HSK 5 grammar — Adverbs.