Direct or indirect object pronoun
 

"solo le he visto una vez" is translated as " I have only seen him once". I just dont understand why this sentence use an indirect object pronoun "le" instead of a direct object pronoun "lo".
for example: I have seen him, would be "lo he visto"

Reading up on leísmo will answer your question.

Thank you Rusty. What about this one "¿Ya se ha ido?"? Is there another odd rule out there that substitute "se" for "lo" as well?

The pronoun 'se' has many uses, as you'll discover on your journey.
One of its many uses is that of 'reflexive pronoun' (which is used with both reflexive verbs and pronominal verbs).

I believe your example is asking, "Has she/he left yet?"
In that case, you'd be dealing with a pronominal verb. The infinitive is 'irse'. The affixed pronoun 'se' is a reflexive pronoun.

When 'irse' is conjugated, the suffixed 'se' is relocated, and assumes a form that agrees in person with the subject, as does the verb's ending.

This particular conjugation is the 'present perfect', so there is a conjugated auxiliary verb preceding the participle 'ido'.

IRSE - Present Perfect Tense

Person	Singular	Plural
1st	me he ido	nos hemos ido
2nd	te has ido	os habéis ido
3rd	se has ido	se han ido

@Sang: About the uses of "se", you might find this discussion useful:
http://forums.tomisimo.org/showthread.php?t=13735

Actually, if it was translated as "Has she/he left yet?" It would have make perfect sense to me. But it was translated as "has it already gone?" And that "it" is what caused me to be confused as of why the sentence use "se" instead of "lo".

The pronoun 'it' is also third person, and it's a subject pronoun, not a direct object.

The question they proposed and the question I proposed are both said the exact same way. Both use a third-person subject pronoun, which is not provided in the Spanish question.

Since no Spanish subject pronoun was specified, it could be any of the third-person English subject pronouns (which also includes the Spanish usted).
And it doesn't have to be a pronoun either. Any topic known to the speakers could have been inferred.

The Spanish subject pronoun 'ello/a' (it) is seldom used. Why it was even hinted at in your study material is beyond me (unless you're using a machine translator).

There's no reason to always provide a subject in Spanish. English requires one, but Spanish does not. Only if your audience has no idea who you're talking about should a subject be provided.

Thank you for the clarification Rusty! I think the sentence might have been translated using machine as well.