以 / 已: how to tell them apart

Both yǐ: 可以 and 以为 use 以; 已经 uses 已.

以 yǐ — by means of (可以, 以为)

以

可以	kě yǐ	can; may	1A lesson 3
所以	suǒ yǐ	therefore	4A lesson 6
以身作则	yǐ shēn zuò zé	to set an example by one's own conduct	4A lesson 8

已

已经

yǐ jīng

already