地: manner (adverbial + 地 + verb)
助词 / particles (结构助词)
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
Adj / reduplicated Adj + 地 + Verb
地 (de) turns an adjective into a manner adverbial placed before the verb, like "-ly" in English: 认真地看 = look carefully.
It's mostly used with two-syllable or reduplicated adjectives: 慢慢地, 高高兴兴地, 认真地, 仔细地.
The three "de" in a nutshell: 的 + noun (我的书), 地 + verb (认真地学), verb + 得 + adj (学得好).
Examples
买东西以前，你要认真地看评价。
Mǎi dōngxi yǐqián, nǐ yào rènzhēn de kàn píngjià.Before buying, read the reviews carefully.
他高高兴兴地打开了快递。
Tā gāogāoxìngxìng de dǎkāile kuàidì.He happily opened his parcel.
请慢慢地说。
Qǐng mànmàn de shuō.Please speak slowly.
孩子们安静地坐着。
Háizimen ānjìng de zuòzhe.The children are sitting quietly.
Common mistakes
✗ 认真的学习 (in a verbal sentence) → ✓ 认真地学习.
✗ 他说地很快 → ✓ 他说得很快 (after the verb = 得).