Well, you can actually. You don’t always have to, but there’s no rule saying you can’t. But what exactly is an infinitive, and what does it mean to split one?
The infinitive form of a verb is basically the standard form of the verb, without inflection or conjugation. It’s the form that strictly should be used when referring to a verb, but not using it in a sentence. English is slightly unusual in that the infinitive form is made up of two parts: the word to, and the base form of the verb. The infinitive form of the verb to eat for example is: to eat. Compare this with Latin (manducare), Italian (mangiare), and French (manger), which, like most languages, have infinitive forms made up of only part.
If we only ever used the infinitive form of a verb simply when we’re referring to it, it wouldn’t be such a problem. In a sentence like…
My favourite verb is to eat.
… we don’t need to worry about splitting the infinitive. But we use infinitive forms of verbs in everyday language for a variety of grammatical reasons. If a verb comes after certain other verbs, for example, we need to use an infinitive form:
I expect to arrive soon.
Or, if we’re explaining why we do something:
I’m going to the shop to buy some milk.
So we do need to use infinitive verb forms sometimes. And sometimes when we do, we want to give some detail about the action we’re using the infinitive form to describe. Then, we can use an adverb, and this is when we might want to split an infinitive. For example, all the following sentences and phrases contain split infinitives:
I’m going to the shop to quickly buy some milk.
Every morning I like to quietly clean the kitchen.
To boldly go where no man has gone before.
We can see how in these examples we’ve put an adverb between the two parts of the infinitive. Before we look at how to change these examples, why would someone have a problem with splitting an infinitive in the first place? The main argument against doing so is that an infinitive form of a verb is a single grammatical unit, and therefore shouldn’t be split up. This is probably a holdover from people learning Latin in school. And because in Latin you can’t split an infinitive, and because Latin has always been revered, people decided that you therefore shouldn’t split an infinitive in English either.
If you believe that, then you’d have to change those examples above:
I’m going to the shop to buy quickly some milk./I’m going to the shop quickly to buy some milk./I’m going to the shop to buy some milk quickly.
Every morning I like quietly to clean the kitchen./Every morning I like to clean quietly the kitchen./Every morning I like to clean the kitchen quietly.
To go boldly where no man has gone before./Boldly to go where no man has gone before./To go where no man has gone before boldly.
Putting the adverb at the end of the sentence often works OK, but in a long sentence it can be hard to keep track of which verb the adverb is describing. And in a sentence with multiple verbs, putting the adverb at the end can be confusing. If we say to go where no man has gone before boldly, does boldly refer to the to go at the start, or to the has gone near the end? It could be either, so the phrase could also mean that we’re going to a place where people have gone before, but they just haven’t gone there boldly before.
I think most people will agree that putting the adverb before to sounds a bit clumsy, and can also be confusing. If we’re using two verbs, it could be understood to refer to the first verb (e.g. I like quietly to clean the kitchen – does quietly describe the way we like it?)
There are other situations where not splitting the infinitive could lead to confusion of meaning. For example:
You have to really think about it to figure it out!
This emphasises that you have to think hard in order to figure it out.
But if we don’t want to split the infinitive, we could say:
You really have to think about it to figure it out!
This is subtly different, and merely emphasises that it’s necessary to think about it to figure it out, but not necessarily think hard about it.
So rather than getting yourself into such confusion, why not just split the infinitive? Doing so never really causes your meaning to really be unclear, and you can always put the infinitive back together again and use it later!