Learn English – the use of “can be” in passive voice

The idea of "converting" an active to a passive and vice versa is something of a misnomer. You can create pairs of active/passive sentences that are approximately equal in meaning, but probably never exactly equivalent.

One reason for this has to do with information theory, in other words, the types of things/information that we tend to "encode" in particular syntactic positions in a sentence. In the sentence "I gave Dave the book", I am focussing slightly on my action of giving compared to "Dave was given the book", where I focus slightly on the impact of David receiving the book. This means that it is odd to "convert" e.g. "The bus left the station" into "The station was left by the bus", because we don't conceptualise the movement of a bus from a station as having an "impact" on the station. (Other cases where the passive would be odd include e.g. "A headache was given to Jane", "The prime minister was become by David Cameron", "10 is equalled by 5+5"...)

Another reason has to do with scope effects. If I say "Chocolate is eaten by children", this tends to imply that (a) children are the only/principal entities to eat chocolate and (b) children may eat a range of other things. But if I say "Children eat chocolate", this tends to imply that (a) children primarily eat chocolate over other food, and (b) does not imply that children are the only/main entities to eat chocolate.

I believe your confusion is that the verb applied is indeed intransitive, but this is a phrasal verb that is actually being used here. This is indeed correct below.

Samples that you requested have been applied FOR.

What is happening is that this is the phrasal verb-

Apply for

Not

Apply

Please refer to this entry in the Free Dictionary about this phrasal verb.

A preposition at the end of a sentence in this example is correct because for is a particle of the verb phrase...