As or like?

As and like are prepositions or conjunctions. The prepositions as and like have different meanings. As + noun means ‘in the role of’, like + noun means ‘similar to’ or ‘in the same way as’.

Compare

As your father, I’ll help you as much as I can.	The speaker is the listener’s father.
*Like* your father, I’ll help you as much as I can.	The speaker is not the father but wishes to act in a similar way to the father.

We use like (but not as) to compare two things:

She’s got a headache like me.
Not: ~~She’s got a headache as me.~~
Like the other students, he finds it a bit difficult to get to lectures early in the morning.
Not: ~~As the other students, he finds it~~ …

When we compare appearance or behaviour, we use like, not as:

That house looks like a castle.
Not: ~~That house looks as a castle.~~

As is commonly used to talk about jobs:

He worked for a long time as a teacher in Africa.
Not: … ~~like a teacher in Africa.~~

The conjunctions as and like have the same meaning when used in comparisons. Like is a little more informal.

Nobody understands him as I do.
Nobody understands him like I do.