Active or passive infinitives

be + to be seen/found/congratulated (He was nowhere to be seen)

The reason behind his actions is impossible to understand. / Isn't it difficult to make yourself understood in a foreign language?

No one was to blame.

Let/allowed to

It's only to be expected.

modal verb + be + past participle (The castle must not be destroyed)

modal verb + have + been + past participle (The secret should have been kept.)

obligation

there is

something/anything/nothing

to blame

If the subject of the sentence is the person who has to do the action, the active infinitive is used. She has got letters to write.

If the subject of the sentence is the action that has to be done, we use the passive infinitive. These shoes are to be washed.

There's nothing to do - I'm bored

There's nothing to be done - There's no way of putting it right.

make, hear, see, help

They made me cry. I was made to cry.