Si in a ke clause, and each other

Inside a ke clause

Si looks at the subject of its own clause, not of the whole sentence:

In the second row, the brother is the subject of the ke clause, so sia is ruled out. In the third, the clause's subject is mi, which never uses si, so the man who asked is lin.

Infinitives: say it with ke

An infinitive has no subject of its own, so the rule has less to grip:

• Li petis la knabon helpi sin. - He asked the boy to help himself.

The usual reading takes sin as the boy, the one doing the helping, but not every reader parses it that way. A ke clause leaves no doubt:

• Li petis, ke la knabo helpu sin. - He asked the boy to help himself.
• Li petis, ke la knabo helpu lin. - He asked the boy to help him.

Each other, and si mem

With a plural subject, sin can mean themselves or each other:

• Ili amas sin. - They love themselves. / They love each other.

Context usually settles it. When it does not, the reciprocal has its own phrase:

• Ili amas unu la alian. - They love each other.
• Ili parolis unu kun la alia. - They talked with each other.

Unu never changes; alia takes whatever its job needs: -n as the object, nothing after kun.

Mem (self) adds emphasis. With sin it stresses the object; with a subject it stresses who did it, and has nothing to do with the reflexive:

• Li vidis sin mem. - He saw himself (and nobody else).
• Li mem faris tion. - He did it himself.

Reach for -iĝ- before sin

Where French or Spanish would use a reflexive verb, Esperanto usually has -iĝ-:

For things, -iĝ- is the natural choice: la pordo malfermis sin pictures the door opening itself, as if it acted. For a person moving deliberately, li levis sin is also correct and common. Keep sin where the subject really acts on itself, as in li kaŝis sin. The -ig- and -iĝ- article has the system.