How to tell when an adjective can be used as an adverb?

Question

In conversation with a native German speaker I said,

Er las beschäftigt.

She said that was incorrect and it should be,

Er war beschäftigt zu lesen.

To investigate this I asked Google translate for the following translations:

He was busy reading

He was busily reading.

He read busily.

Google replied:

Er war beschäftigt zu lesen

Er war fleißig am Lesen.

Er las eifrig.

When I queried Duden about beschäftigt, fleißig and eifrig it indicates they are all adjectives.

So my question is, while English uses the same words to express similar ideas in various ways, I do not understand why German requires different words to do the same thing in these cases.

Answer 1

The problem here is not so much about using a past participle as an adverb (even if "beschäftigt" is often listed as an adjective, it's still the past participle of "beschäftigen"), but rather transitivity and reflexitivity of this special verb:

beschäftigt can be translated as "busy" when

• used reflexively with "mit" ("zu" is [marginally] possible, but very uncommon): Er beschäftigt sich mit Lesen
• used in an intransitive way with "mit" ("zu" doesn't work here): er ist mit Lesen beschäftigt
• used in a transitive way where the actor is the object: Lesen beschäftigt ihn - this rather translates to something like "Reading keeps him busy"

(Note how German prefers he nominalization of the verb here)

Your translation doesn't fall into any of those categories (you simply can't form the "mit" and/or the reflexitivity into the participle/adverb). "Er las beschäftigt" would be understood as not connected between the "busy" and the "reading" part - as something along the lines of "he was reading and employed".

The Google translation of "he was busy reading" to "er war beschäftigt zu lesen" is - if not completely wrong - at least doubtfully uncommon (and sounds very much an anglicism to a native speaker). DeepL translates the same sentence to the very much better "Er war mit Lesen beschäftigt"

So, as a general rule

when a verb requires a preposition to express what you mean, the participle of that verb might be difficult (even impossible) to use as an adverb.

Answer 2

I wanted to comment on the English half of this translation a bit, since tofro's answer already explains the German half. While English uses the present participle/gerund (the "-ing" form) in a number of ways, German only uses it to form an adjective, and even then the adjective must go in front of the noun. That means that many English sentences involving "-ing" can be a challenge to translate into German. You generally have to rephrase the English before you can get anything sensible in German. In this case, the main content of the sentence is "He is busy" -- Er ist beschäftigt. To figure out the role of "reading" here, think about what question the "reading" part would answer. I think it would go something like this:

How is he?
Busy
What is he busy doing?
Reading

So it's not "busy" that's modifying "reading", in which case you'd use "busily", but "reading" that's modifying "busy". But recall that German does not use the "-ing" form this way; it's more natural to rephrase the question to ask for a noun:

What is he busy with?
Reading

To make a verb into a noun in German you just have to capitalize it: Reden. You have to use a preposition to connect Reden with lesen. The question "What is he busy with?" gives you a clue that the preposition required is mit.

Keep in mind that while German grammar is difficult, English grammar is also difficult, but if you're a native English speaker you tend not to notice. So while you might think that knowing German grammar is enough, many times you also have to know enough English grammar to properly analyze an English sentence before trying to translate it. English and German are both Germanic languages, so their grammars are more similar than you might expect. But both have features not present in the other and that fact can make translation tricky in either direction.