When should "like" be used rather than "as" in a comparison?

Question

In Italian, "spies like us" becomes spie come noi, and "do as you like" becomes fai come preferisci.
In both the sentences, the translation of like, and as is come. This causes some problems to the Italian native speakers, who tend to use the English like instead of the correct word.

In which cases is it correct to use like?

I am not referring to like used as verb, but to like used as preposition/conjunction. For example, is it correct to use like in the following sentences? I am interested in a general answer, not an answer limited to the following examples.

• Do like we do.
• Like you wish.
• She is tall like your sister.
• She did like you said.

Answer 1

In my opinion, none of the above sentences are correct.

Like can be a preposition, and is used before nouns and pronouns to talk about similarity. For example:

He ran like the wind.

or

A person like you knows that..."

As, on the contrary, is a conjunction. It is used before a clause and before an expression beginning with a preposition. For example:

Nobody knows her as I do.

or

In 1939, as in 1914, everybody seemed to want war.

It is true, however, that in informal English, like is frequently used as a conjunction instead of as, so the first sentence "Do like we do." could be heard.

The third sentence is wrong because you are making a comparison and in this case as is the only one you can use. If on the contrary the concept which you want to convey is that the girl is tall and that your sister is tall too, then like is perfectly fine, but you need a comma to separate it from the first part of the sentence (that is, "She's tall, like your sister.").

Finally, you normally also use as when talking about the function or the role of something. For example:

He works as a waiter.

or

Don't use that knife as a screwdriver.

Disclaimer: The grammar explanations and most of the examples are taken from Swan's "Practical English Usage".

Answer 2

When comparing attributes, use "like":

• Spies like us.

  There are many spies, only some of them are similar to (or like) us.

• She is tall, like your sister.

  Both your sister and "she" are tall. She is tall as your sister would also be a correct sentence, but it means that she is equally tall, or possibly taller--not just they are both "tall."

In your other examples, the two words are synonyms, although "like" is less formal--possibly even technically incorrect, but widely understood.

• Do like we do. / Do as we do.
• Like you wish. / As you wish.
• She did like you said. / She did as you said.

Answer 3

That question is difficult to answer as the function word like is extending its use, probably because "as" has too many uses and a lot of people don't want to analyse whether a preposition or a conjunction is needed. Here language is moving. But in correct language "like" is an adjective, used as a preposition and "as" is a conjunction. But "as" can also be a preposition as in "As a young man, Eliot had studied art in Paris". Here "like" would make no sense.