I was confused about “to be + past participle"

Question

Why does the following sentence use "to be reinforced" rather than "was reinforced"

The fact that organisms evidently inherit the capacity to be reinforced by certain kinds of event does not help us in predicting the reinforcing effect of an untried stimulus.

and in what case should I use to be + past participle?

Answer 1

The problem is that you didn't parse the sentence correctly.

The paragraph you presented does not say that "some capacity was reinforced by certain kinds". What it does say, actually, is that "the capacity to be reinforced by certain kinds of event ..."

Therefore "the capacity to be reinforced" can be rewritten to:

The ability to be reinforced.

The same way as you may have:

The ability to swim.
The ability to walk.
The ability to be resistant to cold.
The ability to be steady.

etc

So, the original sentence may be rewritten to:

The fact that organisms evidently inherit the ability to be reinforced by certain kinds of event
[now starts a whole another sentence]
does not help us in predicting the reinforcing effect of an untried stimulus.

A much more simplified alternative example of the above sentence would be:

I inherited my ability to sing from my father.

Does this help you to understand what's happening there?

Answer 2

In addition to what Rimmer has said, 'the capacity to be reinforced by certain kinds of event' is a passive construction. That is one of the main uses of ‘be’ + past participle.