The Use of Infinitives (OF PURPOSE). Thanks!

[make_wishes_2010s]

On page 89 of the 5th ed Sentence Correction, under the discussion on Verb Modifiers, the textbook stated that "Infinitives of purpose can be used with unnamed agents in passive-voice sentences" and then gave an example considered as grammatically correct:

The weight was lifted to free my leg.

However, on page 255 under the further discussion on Infinitives, the following sentence was considered "wrong":

The building was demolished to avoid falling down accidentally.

This sentence used passive-voice, right? So what are the structural differences between the two sentences above?

What is more, we know that verb modifiers can be placed more freely than none modifiers; however, I don't know how "freely" can I place the "Infinitives of purpose". For example, on page 255, the following sentence was used to correct the sentence above:

The building was demolished to keep it from falling down accidentally.

Then I'm wondering if the sentence can be changed to the following and still be considered correct:

To keep it from falling down accidentally, the building was demolished .

Or, using the first example, can I change it to:

To free my leg, the weight was lifted.

and still be right?

Thanks!

[RonPurewal]

This is not a grammar issue at all. It's an issue of whether the particular "purpose" makes sense without being assigned to an explicit agent.

In considering "to + verb" to express a purpose, the following distinction exists:

* Some purposes exist independently, without reference to whose purpose we're talking about.
e.g.,
to conserve water
to prevent injuries

* Other purposes are nonsense without specifying who is trying to achieve them.
e.g.,
to get better grades
to avoid falling
to lose weight

If something is in the first category here -- i.e., it actually makes sense without a reference to who or what is doing the thing in question -- then you can write the whole sentence without an agent.
Aerators were installed in the faucets to conserve water.
A soft surface was laid over the pavement to prevent injuries.

If something is in the second category -- i.e., nonsense unless you know who/what is trying to achieve the stated goal -- then you can't omit the agent, because then it's nonsense.
e.g., This material should be studied to get better grades --> nonsense. Who's getting better grades? "Getting better grades", unlike "conserving water" and "preventing injuries", is a personal goal that cannot be divorced from the person who's trying to accomplish it.
Here, you'd have to write You should study this material to get better grades. Or Carla should study this material to get better grades. Or whatever.

[RonPurewal]

Looks like you edited this post while I was writing. (The "to keep it" stuff wasn't there when I wrote my original reply.)

The building was demolished to keep it from falling down accidentally.

Now you've got a purpose that doesn't need an agent anymore. "To keep the building from falling down" is like "to conserve water" or "to prevent injuries" -- it's no longer essential who is trying to do it.
For instance, you can also write
The retrofitting equipment was installed to keep the bridge from falling down in an earthquake.

To keep it from falling down accidentally, the building was demolished .

This sentence is the same as the preceding one. The order of the parts doesn't affect the meaning.

There are rhetorical differences -- i.e., switching the order places the emphasis of the sentence differently -- but rhetoric is not tested on SC.
(In fact, it couldn't possibly be tested, because a single sentence doesn't give enough information to determine what deserves extra emphasis).

Or, using the first example, can I change it to:

To free my leg, the weight was lifted.

Same as what's there. Works.

[ImranM766]

Dear all:

Within the parameters of Ron's explanation, why is (A) incorrect and (B) correct?

(A) Coffee was drunk by her to stay awake.
(B) The book was short enough to read in one night.

Would be it suitable to say that the infinitive of purpose is an adverb? As such, it modifies the subject and verb, requiring an agent for specific causality. Even then, why is A incorrect? Thank you for your review.

[esledge]

Within the parameters of Ron's explanation, why is (A) incorrect and (B) correct?

(A) Coffee was drunk by her to stay awake.
(B) The book was short enough to read in one night.

Would be it suitable to say that the infinitive of purpose is an adverb? As such, it modifies the subject and verb, requiring an agent for specific causality. Even then, why is A incorrect? Thank you for your review.

Yes, I'd say the infinitive of purpose acts like an adverbial modifier. However, it would just need to modify a verb, not necessarily a subject and verb. In fact, my first thought was that (A) is wrong because "by her" gives the subject, so why use the passive voice verb at all? Changing to active voice also lets you put "to stay" closer to the verb, where it makes better sense:

Corrected (A): She drank coffee to stay awake.

Note that "coffee" is still in between the infinitive and the verb, but it's short and not misleading.

So I am thinking of this part of Ron's explanation:

This is not a grammar issue at all. It's an issue of whether the particular "purpose" makes sense without being assigned to an explicit agent.

In your example (A), there actually was an explicit agent ("her"). Your example (B) was a better example of no explicit agent:

(B) The book was short enough (for anyone) to read in one night.