How do you distinguish prepositional phrase from idioms?

[susumuya]

I find it hard to distinguish the two of them.
Are there any ways to distinguish prepositional phrase from idioms?

How can we categorise idioms into various subtypes?
Like, fall in love (with sb.), run a mile, work out, beside onself, dress up...

If 'keep up sth.' is an idiom, is it still a prepostional phrase?
If yes, why some prepositional phrases are idioms and some are not?

Thanks!!!

[Tdol]

There's debate about prepositional phrases, and it depends on people's interpretation. The Cambridge dictionary of idioms, for instance, takes a wide view and happily includes them. We do too on the site, on the grounds that they might cause comprehension difficulties for learners. With phrasal verbs, some people use the term 'idiomatic', which is one way of avoiding the problem, but it can be difficult to say define things exactly:
The building's on fire.
The player's on fire. (performing well)
Here you can see a difference in meaning, with the second being an idiomatic use. Does that make 'on fire' a prepositional phrase or an idiom?

BTW, I am not sure about the categorisation you refer to.

[paulmoss]

Dear susumuya,
What distinguishes idioms from other types of phrases is the fact that their meaning can't be worked out even when you know the meaning of individual words. Most of the time, the meaning can't be guessed from the context either.So fixed expressions and phrasal verbs can be literal, or idiomatic. Take these for example:
"They took him off to the police station." ( an example of a literal phrasal verb; if you don't know the meaning you can easily guess it from 'take' and 'off')
"John can take off the Prime Minister very well." ( If you don't know this one already, it will be really hard to guess. (I should say impossible)