地 (de) turns a word like 高兴 (happy) into something like "happily". It goes between the description and the verb, so it tells you how someone does something. It is an HSK 2 grammar point and one of the three particles pronounced de.
Subject + description + 地 + verb.
The description comes before the verb, not after it as in English: "he said happily" is 他高兴地说, literally "he happy 地 said". 地 is pronounced with a light, neutral tone. If the description is negative, 不 stays inside it: 不高兴地 means "unhappily".
A one-syllable adjective is often doubled before 地: 慢慢地 (slowly), 好好地 (properly, well), 早早地 (nice and early). Doubling makes the description sound more vivid.
With doubled words, 地 can usually be left out. 你慢慢吃 and 你慢慢地吃 are both correct.
A single one-syllable adjective does not normally take 地. Put it straight before the verb (快走, 慢走), or add 很 and then use 地: 很快地. 快地 on its own sounds wrong.
地 also follows counting phrases such as 一个一个地 (one by one) and 一个字一个字地 (one character at a time).
All three are pronounced de, so learners mix them up in writing. 的 comes before a noun (快乐的孩子, a happy child). 地 comes before a verb (快乐地笑, laugh happily). 得 comes after a verb and adds a comment on how the action is done (跑得很快, runs fast).
A simple test: look at the word straight after the gap. If it is a noun, write 的; if it is a verb, write 地. If the gap comes right after the verb, write 得.
The most common mistake is writing 的 instead of 地: 她高兴的说 should be 她高兴地说. You will see 的 used this way in casual writing online, but in textbooks and in the HSK exam the verb needs 地.
The other mistake is putting the description after the verb, as in English. 孩子们唱歌快乐地 is wrong; say 孩子们快乐地唱歌. The description with 地 always comes before the verb.
Practise this grammar point on paper: 20 exercises using only HSK 2 words, with an answer key on a separate page. Free to print and copy for your class.
Every example on this page uses only words up to HSK 2. Paste your own sentences into the level checker to see which words, if any, are above your level.