What is the grammar rule for the preposition after the word free? Why do we say free of impurities but free of charge?
Learn English – grammar rule for prepositions
prepositions
Related Solutions
For the most part, your book is correct, but you are right to be suspicious. The appropriate word can change depending on the intended meaning of the sentence in some cases. Here are a few examples:
I have had a change of heart
It is a change for the betterWhat we have here is a failure to communicate
This is a failure of epic proportionsI have the ambition to succeed
I have an ambition for greatness
Note: it is a good idea to talk to your instructor about questions like this. You may not be required to know all of the special cases right now. Special cases can make the learning process more complicated, so some classes will save them for later.
Wikipedia has this to say:
In ambiguous cases, there is not always a clear rule that dictates which adposition is appropriate, and different languages and regional dialects may have different conventions; the standard usage(s) of a given preposition can be idiomatic. Learning the conventionally preferred word is a matter of exposure to examples. For example, most dialects of American English have "to wait in line", but some have "to wait on line". It is for this reason that prepositions are one of the most difficult aspects of a language to learn for non-native speakers.
For reference, here is the description of adposition used in the same article:
In more technical language, an adposition is an element that ... indicates how that phrase should be interpreted in the surrounding context. Some linguists use the word preposition instead of adposition for all three cases.
The informal rule is a stylistic one. Keep the complement as close as possible.
That really helps me out.
Clearly this is not a lot of separation, and to phrase it "helps out me" would sound awkward and awful.
That really helps out the children who are starving every day in Africa.
To put "out" at the end would simply require the reader or listener to wait too long to parse your verb as a phrasal verb.
To sum it all up: it's a judgment call.
To sum up everything I have stated in this response: it's still a judgment call.
Best Answer
The "charge" vs. "impurities" difference you actually seem to be asking about is nothing to do with the preposition as such; it's to do with "charge" being a "mass noun", so not calling for pluralization, and "impurity" being a "count noun", which it's appropriate to pluralize here.
This distinction is the same one as produces the "less vs. fewer" dichotomy.