作为: as, in the capacity of
【四18】介词 / prepositions
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
作为 + a role noun = "as, in the capacity of": it says in what capacity someone acts, speaks or is judged. The phrase comes first (followed by a comma) or right after the subject: 作为学生，你应该按时完成作业; 他作为教师代表参加了这次会议.
The noun that follows is a role or status (学生, 朋友, 父母, 工程师, 志愿者), and the rest of the sentence often says what that role requires or allows (应该, 要, 可以, 一定). Register: neutral to formal, very common in speeches and the press.
Don't mix it up with 对……来说 ("for", how a thing looks to someone: 对学生来说，这很难) or with the spoken 当 (当老师不容易). Before an object, 作为 can also mean "as, by way of": 把这本书作为礼物送给他.
Examples
作为学生，你应该按时完成作业。
Zuòwéi xuésheng, nǐ yīnggāi ànshí wánchéng zuòyè.As a student, you should hand in your homework on time.
作为朋友，我一定会帮你。
Zuòwéi péngyou, wǒ yídìng huì bāng nǐ.As your friend, I will definitely help you.
他作为教师代表参加了这次会议。
Tā zuòwéi jiàoshī dàibiǎo cānjiā le zhè cì huìyì.He attended the meeting as the teachers' representative.
作为一名志愿者，她每个周末都去帮助老人。
Zuòwéi yì míng zhìyuànzhě, tā měi ge zhōumò dōu qù bāngzhù lǎorén.As a volunteer, she goes to help elderly people every weekend.
作为公司的经理，他不能只想着自己。
Zuòwéi gōngsī de jīnglǐ, tā bù néng zhǐ xiǎngzhe zìjǐ.As the company's manager, he can't think only of himself.
他把这本书作为生日礼物送给了我。
Tā bǎ zhè běn shū zuòwéi shēngrì lǐwù sòng gěi le wǒ.He gave me this book as a birthday present.
Common mistakes
✗ 作为孩子的时候，我很怕黑 → ✓ 小时候，我很怕黑 (作为 gives a role, not a period of life).
✗ 我作为你一样关心他 → ✓ 我像你一样关心他 (a comparison takes 像, not 作为).
✗ 作为学生，这个问题很难 → ✓ 对学生来说，这个问题很难 ("difficult for…" is 对……来说; 作为 announces what the person does or should do).