Is “to” always necessary?

Question

Will the meaning of this sentence change if I remove "to" in the second sentence?

He wanted to go to the park by himself. But I didn't allow him to.

Answer 1

Strictly speaking, "I didn't allow him" lacks a connective topical element, hence making the topical focus fully on the presented subject point [ie, him]. In short, you'd be stating that you're not allowing that individual's existence.

So, from the perspective of proper sentence structure, it's not appropriate to make such an omission (which should relate to why there's a favoring for the trailing to and alternative forms such as "let him" among the general population) and, in certain rare circumstances, such an omission could in fact cause confusion. As someone who is extremely literal minded, I've had confusions on such matters rather often, in fact, so I can personally attest to the possibility, even if not to the frequency across the general population.

That said, for that particular example and, indeed, for most examples, both context and typical expectations of meaning are going to make it so that proper communication will be achieved without significant issue.

The fundamental worth and premise of language is to convey intent and meaning as clearly as possible (well, perhaps excepting its usage within poetry). While we can certainly state that this passes a minimum bar, and thereby meets more general expectations of what language should accomplish, the "as clearly as possible" aspect isn't going to be met, if there's any chance of unnecessary miscommunication (regardless of how unlikely it may be).

So, in short: Yes, it's fine for conversational English (and such an omission may even feel more natural there at times), where an avoidance of miscommunication isn't held in strict demand (though, if you've watched or read enough dramas involving people leaping to misunderstandings over simple confusions, you can certainly see the value even there), but I'd definitely avoid any lack of clarity in any technical, business, or literary work. [Though, as Adam Pendragwn's response to this topic suggested, it may in fact be best to simply combine the sentences when writing to such standards, as ending with a preposition is already rather informal to begin with.]

Let me add a bit more clarification as to why it's an issue:

What you're removing in the second sentence directly equates with the following, in your first sentence: He wanted to go to the park by himself. After removing that, you get "He wanted to go the park by himself". Let's change that up a bit, with two alternative examples: "He wanted to travel [to] the park by himself" changes the precise meaning from visiting the park to moving through the park, but conveys similar core intent. "He wanted to advance [to] the park by himself" entirely changes the meaning, suggesting he's promoting the park rather than making his way towards it.

While it may be a bit less clear at first for the sentence with the trailing "to", with the [in]correct formatting, you'll achieve the same end result. While we're not likely to interpret you as having randomly refused someone's existence by your own say-so (unless you're a particular sort of politician), another context may be a bit less intuitive in regards to avoiding any confusion.

This is essentially the entire purpose of indefinite articles in the first place. Pretty much any indefinite article can be removed from a typical sentence without too much effect on the resulting interpretation, but they're nevertheless highly valuable in both helping sentence flow and assisting in ensuring no miscommunication occurs in contexts where interpretation is a bit less intuitively clear.

Simply put, a preposition can be just as effective at fulfilling its role at the end of a sentence as it is in the middle of one. Trailing prepositions may seem unusual given their comparative scarcity of usage, but they're fundamentally no different than any other preposition.

Rather, the confusion here stems from our removal of redundant topical information: "He wanted to go to the park by himself. But I didn't allow him to go to the park by himself." The "to" is effectively acting as a shorthand for a longer sentence. By removing it entirely, you completely lose the intended topic.

At that point, you'd need to combine the sentences [via a comma], as the context is no longer presented in a manner that would properly connect a topic suitably between two seperate sentences.

Answer 2

No, it’s not always necessary. Here’s why: first of all, your example should be one sentence, not two—there should be a comma between 'himself' and 'but'.

Now the sentence reads:

“He wanted to go to the park by himself, but I didn’t allow him to.”

If you’ll notice, the second part of the sentence is not a complete thought—“I didn’t allow him to” what? That makes it also an incomplete sentence, but it’s okay—as long as you’re using the sentence in a casual way, as apposed to, say, writing a formal letter—because one can still understand what you’re trying to express.

Now then, if you had written:

“He wanted to go to the park by himself, but I wouldn’t allow him.”

People would still understand exactly what you meant. So, in a case where you’ve written an incomplete sentence in the first place, yet people can still understand what you wish to express, it doesn’t really matter where you end the sentence—in this case, you can leave off the final “to” because, again, people will still understand what you mean.

Answer 3

Saying or writing "I didn't allow him to." is poor grammar and the answers and comments saying that it's ok are enough to make one cringe.

Saying "I didn't allow him" is correct because it is saying "I didn't allow him [to go to the park by himself]", as well as not ending a sentence with a preposition.

Using "I didn't let him" or "I didn't allow it" are more common formulations, as well as being correct and conveying the same message.

Answer 4

"But I didn't allow him" is not a complete sentence in the way that "But I didn't allow him to" is, and I can't believe that two comments on your question have suggested that it would be acceptable. Maybe strictly there is no change in meaning between the two, but where there is a change is in that one of them (-- with the 'to') sounds like a perfectly natural example of English, and the other sounds to my ear almost like a stereotypical mistake that a non-native speaker might make.

The natural way to phrase it without ending your sentence with 'to' would be:

"He wanted to go to the park by himself. But I didn't let him".