If "to X" signifies an infinitive, what form is the phrase "to be able to X", and can it be split?

Question

"To slowly walk" is a split infinitive which is sometimes frowned upon, in which case "slowly to walk" or "to walk slowly" is often preferred.

"To be able to walk" is a bit more complex, but what are the rules for splitting up this form in order to modify it with an adverb?

"To be easily able to walk"? "To be able easily to walk"? "To easily be able to walk"?

And what happens if it is not "to be able" but another tense, like "was able to"? Ie: "He was able easily to walk"? "He was easily able to walk"?

What is correct and/or preferred?

Answer 1

This is an excellent question.

How do we describe the sentence pattern "[subject] + [form of to be] + able + [to-infinitive]"?

I would describe it this way.

We are using an adjective that can take the infinitive. The word "able" is an adjective.

There are many adjectives that can be complemented by the to-infinitive, such as: able, willing, eager, ready, prepared, happy, proud, pleased. [1]

These adjectives can also be complemented by a split infinitive. (A split infinitive is an infinitive in which an adverb separates the particle "to" and the verb.)

Here are some examples:

She is able to carefully read through the text.
She is able to quickly find the answer.
He was happy to publicly praise the president.
The officers are trained to immediately respond.

So the pattern you mention is the pattern of complementing an adjective with the to-infinitive. There are many adjectives that can be complemented by the to-infinitive, and the adjective "able" is one of them.

We can write this symbolically as SVAI, where S is the subject, V is a form of "to be", A is an adjective ("able"), and I is a to-infinitive.

There are many sentences (as shown) that follow this SVAI pattern. If there's a name for this pattern, it might be this: "adjectives that take the infinitive".

Let's look at one more set of examples:

I was easily able to get there on time.
I was very happy to see them win.

This is not the same as a split infinitive, since the adverb is placed before the adjective. But it shows that there are many different ways of using adverbs in these sentence constructions.

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Footnotes:

[1] https://learnenglish.britishcouncil.org/grammar/english-grammar-reference/to-infinitives

Answer 2

The predicate adjective (be) able is a periphrastic modal. That means it's periphrastic (Greek for "paraphrased"), and it's a modal. Not a modal auxiliary verb, but an idiomatic construction with modal semantics. In this case, be able to is a synonym for can in many constructions:

• He can reach the rim ~ He's able to reach the rim.
• I can't reach him now. ~ I'm not able to reach him now.

... though not in all constructions:

• This can't be the place! but not *This isn't able to be the place!

That's because be able to does not have an epistemic sense, like can't does. The modal auxiliary can itself doesn't even have an epistemic sense, except in the negative (i.e, epistemic can is a Negative Polarity Item):

• *This can be the place!

So, to answer the question, the phrase be able to + Infinitive Verb Phrase is a (higher) Verb Phrase beginning with the periphrastic modal be able, and taking an infinitive complement with to.

Much like the phrases have to + Infinitive Verb Phrase, be going to + Infinitive Verb Phrase, want to + Infinitive Verb Phrase, and used to + Infinitive Verb Phrase, all of which are periphrastic modals, and all of which take infinitive complements, just like real modals.

Answer 3

"To slowly walk" is a split infinitive which is sometimes frowned upon, in which case "slowly to walk" or "to walk slowly" is often preferred.

As far as I'm aware, "slowly to walk" is found only in legalese; readers outside the legal profession will not only find it strange but may even have difficulty understanding it (especially if the context is unhelpful). So I think it's overstatement to say that it's "often preferred".

More generally, the concept of the "split infinitive" is mostly dead outside of legalese. The rule against split infinitives was a made-up one — never a real part of English — and it's become less fashionable to insist on made-up rules of that sort.

That said, "to walk slowly" is perfectly fine, and may sound better than "to slowly walk"; Google Ngram Viewer indicates that it's still much more common. [link]

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"To be able to walk" is a bit more complex, but what are the rules for splitting up this form in order to modify it with an adverb?

"To be easily able to walk"? "To be able easily to walk"? "To easily be able to walk"?

Any of these is fine — assuming that you intend for "easily" to modify "(be) able" — and they're all about equally common. [Google Ngram Viewer link] The version with "be able easily" sounds a bit stilted to me, but honestly the whole thing sounds a bit stilted to me no matter where you put the "easily".

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And what happens if it is not "to be able" but another tense, like "was able to"? Ie: "He was able easily to walk"? "He was easily able to walk"?

What is correct and/or preferred?

"Was easily able to" is far more common. [Google Ngram Viewer link]