Is it "grammatical?"
Your example sentence is:
It busied me for hours.
According to some current grammar sources (which are always subject to change†) your sentence could be termed "ungrammatical" because it uses the verb busy without a reflexive pronoun as its object.
Some accepted references in standard grammar hold that there are a very few verbs in English that we should use only reflexively. If you were taught that busy should only be used with a reflexive pronoun, this may be the reason.
For example, Cambridge Grammar of the English Language (CGEL) presents (on p1488) the following verbs as having "a reflexive as the only (or virtually the only) type of object permitted."
- absent (from)
- avail (of)
To this list, Collins Cobuild English grammar adds:
Collins Cobuild English grammar goes on to tell us:
true reflexive verbs
3.28. Note that the verbs 'busy', 'content, and 'pride' are true reflexive verbs: they must be used with a reflexive pronoun.
He had busied himself in the laboratory.
Many scholars contented themselves with writing textbooks.
He prides himself on his tidiness.
We would not usually say this in English:
I was contented to be home again.
They prided on their beautiful car.
The soldier absented from the battle.
We availed us of some food.
It busied me for hours.
Instead, we would normally say:
I contented myself to be home again.
(or I was content adjective to be home again.)
They prided themselves on their beautiful car.
The soldier absented himself from the battle.
We availed ourselves of some food.
I busied myself for hours (with it).
This doesn't mean that a student of English, or a native speaker, will never employ or encounter usages like the first examples in everyday speech and writing. It's easy to find them with any search engine. What it does mean to a student of English is that the examples above demonstrate a "proper" use of these verbs according to some accepted sources. If you're learning English, it's a good idea to learn the use of these particular verbs as shown in the counterexamples.
† English is always changing, and there is no "official" authority on correct English grammar and usage. Rather, "correct" usage is put forward as an occasionally fractious consensus among
...and finally, and most importantly, the hundreds of millions of normal people who are none of the above!
Over time, spellings, meanings, and usages achieve the status of correct, and the spellings, meanings, and usages are published in dictionaries and grammars. It used to be that a new word, meaning, or usage could be years in this process of discussion, consideration, and finally publication. But because it now takes only a few minutes to publish a revised spelling, meaning, or usage, and because the number of English speakers in the world is growing so fast, what is correct today may be less correct tomorrow. As a student of English, your best bet is to master the current "correct" usages first. Remember this short list of reflexive-only verbs!
Tha above is a paraphrase and expansion of this answer to a question at our sister site ELU.