I think your question will be better answered if you can give us some examples in context so that we have a sense of what you don't understand.
From reading your question, I have no idea. Phrasel verbs, prepositions and idioms are completely different animal from each other.
Based on my interpretation however, I offer you the following:
Phrasal verb are made up of verb+ preposition. Ex: Cheer up, calm down, come up, look down etc...
Prepositions have at least 2 functions that I can think off at this time. They are words that can add or alter the meaning of a verb. Ex: up, down, from, in , out and at etc...
"Look" is to direct your attention to something. "Look up" means to direct your attention upward. By the same token, look down is to mean the opposit of "look-up". However, "look down" is a phrasal verb which means to treat someone with disrespect or one feels that he is more superior then other. Ex: My neighbor looks down on everyone who drives old cars.
Also another function of prepositions is to help depict a more precise and clearer picture by modifying the noun or noun phrase. If I may say so, it offers a direction or location of something that takes place. Ex: She is standing (on her bed), Iwill wait for you (at the bus stop tomorrow morning), The whole class was evacuated (from the school building becasue of the fire alarm)
Idiom: are phrases like: "here today, gone tomorrow", Beauty is only skin deep", "No pain, no gain", "Tomorrow is another day" etc...
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