To show or say that two things are similar in some way, we either use like or as. However, note that like is a preposition - it is used before nouns and pronouns; as is a conjunction - it is used before a clause or an expression that begins with a preposition.
like + noun/pronoun
- He ran like lightning. (not as lightning.)
- Patty frequently chews and eats ice cubes like me. (not as me)
- My days are swifter than a runner; they are like eagles swooping down on their prey.
- Like a dream he flies away, no more to be found, banished like a vision of the night.
as + clause and as + prepositional phrase
- As a cloud vanishes and is gone, so one who goes down to the grave does not return.
- On Thursday, as on Saturday, the meeting will be at 3:30.
- As water wears away stones and torrents wash away soil, so you destroy a person's hope.
It is worth noting that in modern usage, like is often used as a conjunction instead of as. Using like before a clause, however, is common in informal contexts.
- Like a bat out of hell, I'll be gone when the morning comes. (or As a bat out of hell..)
- Nobody knows you like I do. (or as I do)