What is the difference between suus and eius?

Question

What is the difference between the possessive adjective

suus (his, hers, its, theirs)
^{(and its declensions)}

and the genitive, possessive pronoun

eius (of her, of him, of it)?

Can these words be used interchangeably?

Answer 1

All forms of se, including suus, normally refer to the subject of the main clause of the sentence. Eius, however, normally does not refer to this subject, but to someone else. So the two words have different meanings.

Sextus Tarquinius crudelis est. Lucretia praevidet mortem suam.

"S.T. is cruel. Lucretia foresees her own death."

Sextus Tarquinius crudelis est. Lucretia praevidet mortem eius.

"S.T. is cruel. Lucretia foresees his death."

The words suam and eius must be interpreted this way in the examples above; suam cannot refer to Tarquinius, nor can eius refer to Lucretia.

Answer 2

Nunn's An Introduction to Ecclesiastical Latin:

62. Possessive Pronouns are used adjectivally and are equivalent to the Gen. case of the personal or reflexive pronoun.

In the first and second persons the Gen. of the personal pronoun is rarely found in the sense of a possessive pronoun. Phil. 2:12.

In the third person suus is used reflexively, that is when the person or thing to which it refers is the subject of the sentence or clause in which it stands.

Propterea ergo magis quaerebant eum Judaei interficere: quia non solum solvebat Sabbatum, sed et Patrem suum dicebat Deum.
On this account therefore the Jews sought the more to kill him, because he was not only breaking the Sabbath, but also because he said that God was his Father.… Jn. 5:18

When the person or thing referred to by the pronoun is not the subject of the sentence or clause in which it stands, the Gen. of a demonstrative pronoun (generally ejus or eorum etc.) is used as the possessive pronoun of the third person.

Princeps autem sacerdotum Ananias praecepit adstantibus sibi percutere os ejus.
But the chief of the priests Ananias commanded those that stood by him to smite his mouth (i.e. Paul’s mouth). Acts 23:2.

Sometimes where no ambiguity is likely to be caused suus is used in a subordinate clause when the person or thing to which it refers is denoted by the subject of the main clause.

Idem cum Johanne ad nonam horam ad templum adibat, ubi paralyticum sanitati reformavit suae.
He went with John to the temple at the ninth hour, where he restored the paralytic to his health. Tert. de Oratione xxv.

N.B. In Latin, as in French, the gender of a possessive pronoun does not depend on the gender of the word denoting the possessor; but possessive pronouns agree with the nouns which they qualify in gender, number and case, like adjectives.

Sua mater, His mother. Suus pater, Her father.