以免 / 免得: so as not to, lest
连词 / conjunctions
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
Action (precaution)，以免/免得 + unwanted consequence
以免 (written) and 免得 (spoken) introduce what you want to avoid by doing the first action: "so as not to", "lest", "to avoid".
They open the second clause. What follows is negative (a risk), but with no negation word: 以免迟到 = so as not to be late. That's the main trap for English/French speakers.
Examples
出门前请关好窗户，以免下雨时进水。
Chūmén qián qǐng guān hǎo chuānghu, yǐmiǎn xià yǔ shí jìn shuǐ.Close the windows before going out, so the rain doesn't get in.
电池要单独回收，以免污染土地和水。
Diànchí yào dāndú huíshōu, yǐmiǎn wūrǎn tǔdì hé shuǐ.Batteries must be recycled separately to avoid polluting soil and water.
你早点儿告诉他，免得他担心。
Nǐ zǎo diǎnr gàosu tā, miǎnde tā dānxīn.Tell him early so he doesn't worry.
Common mistakes
✗ 以免不迟到 → ✓ 以免迟到 (no negation: 以免 already means "avoid").
✗ 以免 + a desired result: ✗ 多穿点，以免暖和 → ✓ 以免感冒。