As often in English, it is not easy for the teacher to answer the question. Very often, it is easiest to give examples.
“He is not as tall as his father.”
“The Peugeot is nearly as fast as the Mercedes.”
“She is now just as tall as her mother.”
We use “as” when making equal, or modified equal, comparisons.
“She looks like her father.” Like is used here as a preposition. We don’t say, “She looks as her father.”
Describing jobs or roles (as in the theatre):
We often use “as”:
Arthur Fellig worked as a news photographer for many years. (Not like a press photographer.)
Mr Smith acted as United Nations representative to Malawi until last year.
Introducing examples – for instance, examples of behaviour:
Use as with a verb phrase:
“Don’t eat all the chocolates, as you did last week!”
However, many people now use like in colloquial English, although it is considered wrong in literary English.
Example: "“Don’t eat all the chocolates, like you did last week!”
Use like with a noun phrase, not as.
"She enjoys jazz music, especially performers like John Coltrane or Fats Waller."
Asking about quality:
What's the weather like in Rome at this time of year?
The answer is "Sunny and cool", not "The weather is like sunny and cool." (but see colloquial uses of "like", where people often say, "The weather is, like, sunny." Here "like" is used as a pause in speech, perhaps to give the speaker time to think.)Quick Test with “as”: identify “as” in these examples: (1) in the role of; (2) comparisons (3) introducing an example:
a) He has served as a border patrolman…
b) "La Dolce Vita" seems as harmless as a Gray Line tour of North Beach at night.
c) He collected big fees as a "labor consultant"..
d) Criticism is as old as literary art.
Quick test with “like”: (1) resemblance; (2) example:
a) A young woman who looked like Alix, with her two children.
b) Bertha Szold was more like Meg, the eldest March girl.
c) Thus ideas like "grace", "salvation", and "providence" cluster together in traditional Christianity.
d) Built upon seven hills, Istanbul, like Rome, is one of the most ancient cities in the world.
(Examples taken from the “Lextutor” concordancer.)
Useful web sites:
Cambridge Dictionary http://dictionary.cambridge.org/
Merriam-Webster dictionary: http://www.merriam-webster.com/
Web Concordancer: http://www.lextutor.ca/concordancers/concord_e.html
These sites are also listed in the right-hand column of this Blog.
I hope this has helped. Please leave your comments on the Tag Board.