In the Third Edition of Practical English Usage, Michael Swan points out that in sentences which employ the structure too [adj] to [inf], we don't normally include the object pronoun whose referent is the subject of the sentence.
Thus in the sentence:
The water is too salty to drink (it).
...the subject of the sentence, water, is the referent (or antecedent) of the object pronoun it, which is the object of the infinitive to drink. In such situations, we usually omit the pronoun.
However, when such structures include for, we do sometimes include the object pronoun.
Thus, in the sentence:
The water is too salty for us to drink (it).
... we can include or omit the object pronoun as we choose.
Mr Swan provides other examples in which the use of the object pronoun is optional:
The radio was small enough for me to put (it) in my pocket.
Those tomatoes aren't ripe enough for the children to eat (them).
Your question, though, asks in which situations you need to add object pronouns, and that is not what Mr Swan addresses. He is more concerned about when we omit the object pronoun. You can think of it like this: use the object pronoun wherever it is necessary to preserve the meaning of a sentence, and omit it in the situations pointed out by Mr Swan. On p. 285, for instance, he writes:
If the noun or pronoun is the object of the infinitive, we do not add an object pronoun after the infinitive.
I gave her a paper to read, (NOT ...
a paper to read it.)
He needs a place to live in, (NOT ...
a place to live in it.)