As and Like

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As
1: 'As' can mean 'because'.

As it was raining, we stayed at home.

2: 'As' can mean 'while' or 'at the same time':

As I was walking down the street, I saw Julie.

3: We can use 'as' to talk about the way one thing is similar to another thing. In this case too, 'as' is a conjunction and needs to be followed by a subject and a verb or by a prepositional phrase. Sometimes we invert the subject and the verb in a formal style.

John loves spicy food, as I do (or 'as do I', more formally).

4: We need to use 'as' with expressions like 'as much as' and by 'as adjective as'. This is also talking about similarity. These expressions can be followed by a subject and a verb or a noun or preposition.

John loves spicy food as much as I do.
Lucy travels as much as me.
She's as clever as her sister is.
London's not as big as Mexico City.

5: 'As' can be used with a noun to show someone's position. This is especially common with jobs. In a similar way, 'as' can also be used to show something's function (what we are using it for). It must be followed by a noun.

She works as a teacher.
Don't use the knife as a screwdriver.

Watch out! You can't use 'like' for someone's real job. You need to use 'as'.

~~I work like a waitress.~~

Like
1: 'Like' can be used to give examples. It means the same as 'for example' and is usually followed by nouns or pronouns.

I love big cats, like lions.
Western European countries like France and Spain have high unemployment at the moment.

2: We can also use 'like' to talk about how one thing is similar to another thing. Here 'like' is a preposition and is followed by a noun or a pronoun.

John loves spicy food, like me.
Tokyo is a busy and exciting city, like London.

When we're talking about how things are similar, we often use 'like' with verbs such as 'look', 'sound' and 'smell'.

She looks like her mother.
It looks like rain.
That sounds like a car.
The kitchen smells like lemons.

Traditionally, 'like' needed to be followed by a noun. However, in modern English, we often use 'like' as a conjunction and so it is followed by a subject and a verb. Some people think this is not correct, but it's very common.

John loves spicy food, like I do.

'Like' vs 'as' for similarity
Often, we can use both 'as' and 'like' to talk about similarity.

I love coffee, like Julie / I love coffee, like Julie does.
I love coffee, as Julie does.

We need to follow 'as' with a clause (a subject and a verb). When we use 'as' for similarity, it's not followed by a noun or pronoun.

~~I love coffee, as Julie.~~

However, when we use 'as' to mean a role or job (it's followed by a noun in this case), then we can't use 'like'. Instead, 'like' is talking about similarity.

As your mother, I'm telling you not to go out now. (I am your mother and I am telling you this in my role as your mother.)
Like your mother, I'm telling you not to go out now. (I'm not your mother, but I am telling you the same thing as she is. I am acting in a similar way to your mother.)

Here's another example.

She works as the manager (= she is the manager).
She works like the manager (= she isn't the manager, but she works in a similar way to the manager).

Try an exercise about 'as' and 'like' here.

As and Like

Perfect English Grammar