Does this use the "sein + zu + infinitive" construction? If so, is "leicht" an adjective or an adverb?

Question

Consider the following sentence: "Das Buch ist leicht zu verstehen" meaning "the book is easy to understand."

My question is: in this sentence, is "leicht" an adjective or an adverb?

Anybody who speaks English might be quick to conclude that it is an adjective, since in the English equivalent, we say "easy" and not "easily." But I see good reasons for it to be an adverb (at least in German), and I'm wondering what native speakers think.

Let us talk about the "sein + zu + Infinitiv" construction. Sentences using this construction in German can often be rephrased using modal verbs, such as müssen, sollen, or können. The following examples are taken from this webpage:

1. Nach der Party ist die Halle aufzuräumen = Nach der Party soll die Halle aufgeräumt werden.
2. Die Verkehrsregeln sind zu beachten = Die Verkehrsregeln müssen beachtet werden.
3. Die Krankheit ist zu heilen = Die Krankheit kann geheilt werden.
4. Am Horizont sind die Alpen zu sehen. = Am Horizont können die Alpen gesehen werden.

It's important to say that the meaning of each sentence, and what modal verb is used in the rephrasing, can depend heavily on context.

Now, let us take the third sentence. If we add the word leicht to the sentence, we get

3. Die Krankheit ist leicht zu heilen = Die Krankheit kann leicht geheilt werden.

In the rephrasing with the modal verb, the word leicht is undeniably an adverb, so I would claim the same is true in the "sein + zu + Infinitiv" construction. The same goes for our original sentence: Das Buch ist (leicht) zu verstehen = Das Buch kann (leicht) verstanden werden.

Question

So, with all that in mind, my question is: in the sentence "Das Buch ist leicht zu verstehen", is "leicht" an adjective which modifies "Buch", or is it rather an adverb which modifies the action of understanding the book? Would you even consider this sentence to be making use of the "sein + zu + Infinitiv" construction?

Answer 1

The piece leicht is an adverb that modifies the infinitive. The part after sein is a predicative.

I like to point out that this particular zu-infinitive in copula phrases is actually a replacement form of the Gerundiv, the third participle German features. Consider:

Die Regel ist leicht anzuwenden.

die leicht anzuwendende Regel

You can see from this example that leicht is an adverb as it also modifies the Gerundiv in the attributive form.

Answer 2

In English examples like "this is easy to understand", the construction features an adjective with an infinitival complement. I think, marginally you get things like "an easy-to-understand book". Here "easy" modifies the noun as an adjective and the rest depends on the adjective.

However, in German, you need a participle construction in such cases: "ein leicht zu verstehendes Buch". This shows that the verb is the head and not the adjective, for the verb needs an adjectival ending when you modify a noun. Therefore "leicht zu verstehen" is a verbal infinitive with an adverbial modifier "leicht" (it is an adjective, but in adverbial function).

The "zu+infinitive" construction is sometimes called a modal passive in German (https://de.wikipedia.org/wiki/Aktiv_und_Passiv_im_Deutschen#Modalpassiv). It is not entirely clear whether "ist leicht zu verstehen" features "ist" as a variant of a passive auxiliary, but possibly so. An alternative view is that "ist" is a copula and "leicht zu verstehen" is a predicative constituent (because sometimes you can switch between "sein" and "bleiben", another copula verb: "das ist zu erledigen / bleibt zu erledigen").