大都 / 偷偷: mostly, secretly
【五04】【五07】副词 / adverbs
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
大都 and 偷偷 are both adverbs: 大都 (dàdōu) says that "most of them" are concerned (他的老朋友大都退休了), and 偷偷 (地) that the action is done "on the sly" (我偷偷送给他一件礼物). Like every adverb they stand before the verb or adjective, after the subject, and never in front of a noun.
大都 needs a plural or collective subject (学生, 朋友, 参加的人) and says that the majority, but not all, is concerned: this is what sets it apart from 都 ("all"). It is read dàdōu, not dàdū ("great capital"). It suits writing and careful speech alike, and 大多 is a near-synonym. To name the majority as a noun phrase, use 大多数: 大多数学生, not 大都学生.
偷偷 describes the manner of an action: it is done so that others do not notice, out of discretion or cunning, and it is not necessarily wrong (a surprise, a present). The particle 地 is optional: 偷偷看 or 偷偷地看. 悄悄 ("quietly") is close, but it stresses silence and softness, whereas 偷偷 stresses hiding.
Examples
他的老朋友大都退休了。
Tā de lǎo péngyou dàdōu tuìxiū le.Most of his old friends have retired.
我们班的学生大都很爱学习。
Wǒmen bān de xuésheng dàdōu hěn ài xuéxí.Most of the students in our class love studying.
参加比赛的大都是女生。
Cānjiā bǐsài de dàdōu shì nǚshēng.Those who took part in the contest were mostly girls.
我偷偷送给他一件礼物。
Wǒ tōutōu sòng gěi tā yí jiàn lǐwù.I gave him a present on the sly.
他偷偷地看了她一眼，脸一下子红了。
Tā tōutōu de kàn le tā yì yǎn, liǎn yíxiàzi hóng le.He stole a glance at her and blushed all at once.
来这家店吃饭的大都是附近的居民，游客很少。
Lái zhè jiā diàn chīfàn de dàdōu shì fùjìn de jūmín, yóukè hěn shǎo.The people who eat at this place are mostly local residents, with few tourists.
Common mistakes
✗ 大都学生喜欢甜的 → ✓ 学生大都喜欢甜的 (大都 is an adverb and goes before the verb; in front of a noun, say 大多数学生).
✗ 我偷偷在家 → ✓ 我偷偷看了他一眼 (偷偷 describes an action done secretly, not a plain state like "being at home").
✗ 我们大都学生 → ✓ 我们大都是学生 (大都 is an adverb: before a noun you need a verb such as 是).