Q: If I start a plant indoors and then move it outside, I can say either “I will harden off the plant” or “I will harden the plant off.” But if I use a pronoun, I can only say “I will harden it off,” not “I will harden off it.” What’s going on here?
A: Your question illustrates a characteristic of many phrasal verbs. By “phrasal verbs” we mean those consisting of a verb plus an adverb (like “bring in”), a preposition (“jump over”), or both (“watch out for”).
The phrasal verb in your example, “harden off,” is one of the verb-plus-adverb kind, a very common type that includes “bring up,” “give up,” “look up,” “hand out,” “take off,” “sort out,” “put on,” “put away,” and many others.
When this kind of phrasal verb has an object, and the object is a noun, the noun can go either in the middle of the phrase (“harden the seedlings off”) or at the end (“harden off the seedlings”).
But if the object is a personal pronoun, it has to go in the middle (“harden them off”), not at the end (“harden off them”).
You can see how this works with similar phrasal verbs. The first three versions are acceptable, the fourth is not idiomatic English:
“Bring in the mail” … “Bring the mail in” … “Bring it in.” But not: “Bring in it.”
“Put out the cat” … “Put the cat out” … “Put her out.” But not: “Put out her.”
“Give up desserts” … “Give desserts up” … “Give them up.” But not: “Give up them.”
(We should add that while personal pronouns can’t go at the end, demonstrative pronouns can: “this,” “that,” “these,” “those.” Nobody blinks when we say things like “Did you harden off those?” or “Please hand out these.”)
Here you may be wondering why the little words in those phrasal verbs (“in,” “out,” “up,” and so on) are adverbs and not prepositions. (Many linguists would call them “adverb particles” or simply “particles.”) Here’s why they aren’t prepositions in these usages.
In sentences like “Bring in the mail” and “Put out the cat” and “Give up desserts,” it’s obvious that “in the mail,” “out the cat,” and “up desserts” are not prepositional phrases.
On the contrary, each of those little words modifies a verb (“bring in,” “put out,” “give up”). And the objects (“the mail,” “the cat,” “desserts”) are objects of a verb, not objects of a preposition.
However, the kind of phrasal verb that consists of a verb plus a preposition behaves differently. (Many linguists would call this construction a prepositional verb to distinguish it from the verb-plus-adverb type.)
Examples of the verb-plus-preposition variety include “look through,” “listen to,” “jump off,” “go around,” “look for,” and others.
This kind of phrasal verb can’t be split by an object. When there’s an object—whether noun or pronoun—it goes afterward.
In this case, the addition of an object (as in “Jump off the bridge” … “Go around the pothole”) creates a prepositional phrase (“off the bridge” … “around the pothole”).
Finally, the third kind of phrasal verb combines the other two—verb plus adverb plus preposition. (This one is sometimes called a “phrasal prepositional verb.”)
Examples include “sit in for,” “put up with,” “watch out for,” “look forward to,” “get on with,” and “bear down on.”
With most of these three-part phrasal verbs, the object, whether noun or pronoun, follows the entire phrase: “A guest host sat in for Dr. Phil” … “He can’t put up with her.”
But a few of these three-part constructions, like “take out on,” “let in on,” and “fix up with,” have two objects, one after the verb part and one after the preposition:
“Take it out on him” … “Fix the couple up with an apartment” … “Let Harry in on the secret.” Again, the objects can be nouns or pronouns.
So that’s the story with phrasal verbs and how they work with objects.
But getting back to “harden off,” it’s a term that dates from the early 19th century, according to the Oxford English Dictionary.
In the OED’s definition, to “harden off” means “to acclimatize (a plant) to cold or outdoor conditions by gradually reducing the temperature of a greenhouse, cold frame, etc., or by increasing the time of exposure to wind and sunlight.”
This phrasal verb is also used without an object (that is, intransitively). Here it means “to become acclimatized through this process.”
So you can say either “I’ve hardened off the plant,” or “The plant has hardened off.”
The verb was first used in writing, the OED says, in Robert Sweet’s book The British Flower Garden (1827):
“When rooted, the glass should be removed from them altogether, to harden them off for transplanting.” Here the verb is used transitively, with the object (“them”) inserted within the phrasal verb.
In the next citation, the phrasal verb has no object. This is from Edward Sayers’s book The American Fruit Garden Companion (1839): “As the weather grows warm … the plants should be placed into a separate frame to harden off.”
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from Grammarphobia http://www.grammarphobia.com/blog/2016/06/phrasal-verbs.html