a) This problem is easy to solve.
b) This problem is easy to be solved.
Which sentence is more natural, a) or b)?
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Sign up to join this communitya) is very much more natural. Phrases like "easy to", "hard to", "complicated to" nearly always take an active verb (focussing on the person doing it) rather than a passive.
a) This problem is easy to solve.
b) This problem is easy to be solved. (ungrammatical)
Sentence (b) is ungrammatical. We need to use sentence (a) here. If the Subject of the main sentence isn't the word it, we can't use passives after the adjective easy.
The adjective easy belongs to a family of adjectives often called TOUGH adjectives. (The Cambridge Grammar of the English Language describes them as adjectives which take hollow clauses).
Let's look at the grammatical example:
This sentence has a Subject this problem. The verb in the sentence is BE. This verb is taking an adjective phrase easy to solve as a Complement.
Inside the adjective phrase easy to solve there is an infinitival construction to solve. This verb phrase is a clause without any overt Subject. Notice that although the verb solve is usually transitive, we can't see any Object here.
Adjectives like easy often take infinitival clauses like this. If we don't state what the subject of the clause is, then we need to guess what it is from the context. Here we understand that the sentence means:
But notice that we also understand that the verb solve has an Object in this sentence too. When adjectives like easy have infinitival clauses like this, the Object of the verb in the infinitival is decided by one of the noun phrases in the main clause. When the verb in the main clause is the verb BE, the Object of the infinitival clause is decided by the Subject of the verb BE:
We understand the sentence like this:
Example (b)
The Original Poster's example (b) has a big problem. The infinitival clause is in the passive. This means that it has no space for an Object any more. The writer of this sentence is trying to use the adjective easy like the adjective keen. The adjective keen is not a tough adjective. When we use adjectives like keen, the Subject of the infinitival clause is decided by the Subject of BE:
Now if the infinitival clause is passive with adjectives like keen , there is still a Subject space in the infinitival clause:
However, as we said, easy is a tough adjective. It doesn't work like keen. The Subject of BE cannot control the Subject of the infinitival clause. It must decide the Object of the infinitival clause. This is impossible in the Original Poster's example (b), because there is no Object!
Here is what our brains try to do with example (b):
This is completely ungrammatical. When we use tough adjectives like easy, the infinitival clause must have a space for an Object after the verb. We can't use passive infinitival clauses after tough adjectives like this.
I must say sometimes it is really difficult to decide whether after a noun an infinitive active or passive is the right thing. In any case I haven't found a rule. I think often both possibilities are good. An example from Animal Farm: When there was work to be done the cat could never be found. I'm sure "work to do" is possible as well.
a) This problem is easy to solve.
b) This problem is easy to be solved.
Sometimes it confuses us. but In my case, I've checked it up like this.
This problem is easy (for people) to solve.
= It is easy for people to solve this problem.
= It is easy that people solve this problem.
Here, you can see 'People solve this problem. (O)' correct, but the below sentence doesn't make sense and is not correct grammantically.
'People is solved this problem.' (X)