Adjectives which take infinitival phrases as complement fall into three camps.
Some adjectives determine our interpretation of the subject of the infinitival clause.
Some adjectives determine our interpretation of the object of the infinitival clause.
Some adjectives don't determine our interpretation of either the subject or the object of the infinitival clause.
Adjectives in the first group are often called control adjectives. Generative linguists would say they involve Equi deletion. For our purposes it isn't important what we call them. Let's look at an example:
- Bob was keen to finish his book.
The adjective phrase headed by keen is the complement of the verb BE. Within the phrase, keen is taking an infinitival clause:
This infinitival clause has an understood subject. The subject must be the same as the subject of the main verb, BE. The sentence means:
- Bob was keen [for Bob to finish his book]
So when we use the adjective keen like this, the subject of the main clause controls our interpretation of the subject of the infinitival clause. Some other adjectives like this are anxious, delighted, eager.
Notice that these adjectives only work this way if we don't overtly mention the subject of the infinitival clause. If we actually mention a different subject, the sentence will still be fine:
- Bob is keen for Mary to finish her book.
The adjective keen only determines our interpretation of the subject if we don't state who the subject actually is.
Lastly notice that we can easily use passive infinitivals with adjectives like keen. Passive infinitivals still have an empty subject slot which can be filled:
- Bob was keen to be kissed by Mary.
- Bob was keen [for Bob to be kissed by Mary].
There's no problem using passive infinitival clauses with adjectives like keen
Some linguists who talk about these adjectives, talk about adjectives which take hollow clauses. Others talk about adjectives that involve tough movement. Again, it doesn't matter what we call these adjectives here.
Let's look at an example:
- The orangutans are difficult to please.
Again we see an adjective phrase functioning as the complement of the verb BE. Again this adjective is taking an infinitival clause as its own complement. This infinitival clause has a vaguely understood subject. The subject of the infinitival clause isn't determined by the grammar. The listener can just mentally fill in the subject with whoever they think is suitable. In the sentence above the subject of the verb please is just people in general. We can reconstruct it like this:
- The orangutans were difficult [for people to please]
If we change the sentence we will get a different understanding of the subject:
- I found the orangutans difficult to please.
This sentence probably means:
- I found the orangutans difficult [for me to please].
It doesn't have to mean that though. It could mean that I found that zookeepers generally found the orangutans difficult to please. The subject of the clause is just determined by our interpretation of the context.
However! I haven't given the complete picture here. The verb please usually takes an object. But the object of the infinitives in the examples above is missing. I should have modelled our interpretation of the example above like this:
- The orangutans are difficult [for people to please ___ ]
Here our interpretation of the object slot there is strictly determined. It's determined by the subject of the matrix verb, the subject of the main clause. So we understand the sentence like this:
- The orangutans are difficult [for people to please them]
Here, our interpretation of the object of the infinitive is determined by the subject of the main verb.
Now notice that if we make the infinitival into a passive, the object slot will disappear. Passive clauses don't have an object slot, usually, because the object of the active version of the clause has been turned into the subject of the passive one. There is no object slot to fill. This means that if we use a verb like difficult with a passive infinitival in a sentence like this, the sentence will be badly formed:
- *The orangutans were difficult to be pleased.
If a native speaker reads this sentence it will hurt their language brain. The speaker obviously wants the orangutans to fill the subject slot of be pleased. The grammar won't let this happen. Our brains are already trying to fill in some kind of object slot after the verb please with the orangutans. This results in a car crash:
- *The orangutans were difficult [for someone to be pleased the orangutans]
Other adjectives like difficult are dangerous, strange, good, bad, impossible.
Some adjectives don't fall into groups 1 or 2. We can't use them as predicate adjectives when they have an infinitival clause as complement. That is to say we can't use such adjective phrases as Predicative Complements. One of these adjectives is the word possible:
- *A Rubik's cube is possible to be done.
- *Pineapples are possible to grow here.
- *Whales are possible to swim.
These sentences are ungrammatical. They are odd because they seem to be verging on the grammatical, but just don't seem to quite work properly.
Extrapositions using dummy it
We can use the adjectives from group three in sentences involving extrapositions. These sentences use the dummy pronoun it.
- It is possible to do a rubix cube.
- It is possible to grow pineapples here.
- It is possible for whales to swim.
Notice that if we state the subject of the infinitival clause it must be preceded by the subordinator for, as in the whales example.
We can also use adjectives from group 2 in extrapositions. When they appear in extrapositions, our interpretation of the object isn't determined, it's understood from the context. I've taken these useful examples from WS2's comments
- After my car accident it was difficult to drive.
Here we understand that I probably find it difficult to drive cars. Notice that because the interpretation of the object slot isn't grammatically determined, we can freely use passive infinitivals here:
- After my car accident it was difficult to be driven.
Note that we don't seem to be able to use adjectives from group 1 in extrapositions:
This only makes sense if we understand it as being an actual thing. It cannot be a dummy pronoun here.
1. Adjectives phrases like those in groups 1 and 2 normally function as predicative complements. Predicative complements can describe either the subject or the object of the verb. I have oversimplified things in the story in the post above. I have said that the subject of the matrix verb determines our interpretation of the subject or object of the infinitival, but what determines this is actually the predicand of the predicative complement, not necessarily the subject. So in Bob was eager to please the adjective phrase eager ... describes Bob and therefore Bob is interpreted as the subject of please. But in Bob found Mary eager to please the adjective phrase eager ... describes Mary and so Mary is understood as the subject of the verb please.
2. Yes, possible and impossible are in different groups. Crazy, isn't it.