Learn Hardcore French: Il a oublié son rasoir; elle lui prête le sien.

Lui is an indirect object pronoun meaning “to him” or “to her.” The verb prêter is used as prêter quelque chose à quelqu’un (“to lend something to someone”), so the person receives an indirect object pronoun: lui (singular) or leur (plural).
Le/La are direct object pronouns (“him/her/it”), which would replace the thing lent, not the person.

How does le sien work, and why not just son?

Le sien is a possessive pronoun meaning “hers/his (one),” and it replaces a noun already known from context. It agrees in gender and number with the thing possessed, not with the owner.

Le sien = masculine singular (here, replacing rasoir).
Son is a possessive adjective that must be followed by a noun (e.g., son rasoir). In our sentence we don’t repeat the noun, so we use the pronoun form le sien.

Could le sien mean “his” instead of “hers”? How do we know?

Yes, le sien can mean “his” or “hers.” It doesn’t show the owner’s gender; it only agrees with the thing possessed. Here, context tells us it’s “hers,” because the subject of the second clause is elle. If you need to remove ambiguity, add emphasis:

Elle lui prête le sien à elle. (She lends him hers.)
Or: Elle lui prête le sien, à elle, pas le sien à lui.

Why is it son rasoir in the first clause but le sien in the second?

Both agree with the noun rasoir (masculine singular).

Son rasoir = possessive adjective + noun.
Le sien = possessive pronoun replacing “her razor.”
French possessives agree with the thing owned, not the owner.

Can I say Elle lui prête son rasoir instead of …le sien?

Grammatically yes, but it’s potentially ambiguous: son rasoir could mean “her razor” or “his razor.” Many readers would assume it refers to the subject (elle), but context is key. Using le sien makes “hers” clearer.

Why isn’t it Elle le lui prête here?

Elle le lui prête = “She lends it to him,” where le replaces the direct object (“the razor”). That’s fine if the razor is clear from context, but it doesn’t say whose razor it is. Le sien explicitly communicates “hers.”

What’s the pronoun order if I do use both object pronouns?

Before the verb (in statements and questions):
me/te/se/nous/vous + le/la/les + lui/leur + y + en + verb.
Example in the past: Elle le lui a prêté.
Negation: Elle ne le lui prête pas.
Imperative (affirmative) switches order and uses hyphens: Prête-le-lui !

Why is the past tense a oublié used instead of the present?

Passé composé (il a oublié) expresses a completed past event: “he forgot.” The present (il oublie) would mean “he forgets” (habitual or current ongoing), which doesn’t fit here. You could also say: Il a oublié de prendre son rasoir.

What’s the difference between prêter and emprunter?

Prêter quelque chose à quelqu’un = to lend something to someone (you give it).
Emprunter quelque chose à quelqu’un = to borrow something from someone (you receive it).
Here: Elle lui prête le sien (she lends him hers). If he were borrowing, it would be Il lui emprunte le sien.

Why a semicolon here? Could I use something else?

The semicolon links two closely related independent clauses. It’s stylistically neat. You could also use:

A period: Il a oublié son rasoir. Elle lui prête le sien.
A coordinator/adverb: Il a oublié son rasoir, alors/donc elle lui prête le sien.
Note: French typography normally inserts a (non‑breaking) space before a semicolon.

Why not la sienne?

Because rasoir is masculine. Possessive pronouns agree with the noun they stand for:

Masculine singular: le sien
Feminine singular: la sienne (e.g., replacing brosse)
Plurals: les siens / les siennes

Could lui be female? I thought “lui” was masculine.

Lui as an indirect object pronoun means “to him” or “to her.” It’s gender‑neutral in that function. So elle lui prête can mean “she lends to him” or “she lends to her,” determined by context.

Is à lui / à elle ever used instead of lui?

Yes, for emphasis or contrast:

Neutral: Elle lui prête le sien.
Emphatic: Elle prête le sien à lui, pas à moi.
However, in ordinary, non‑emphatic sentences, use the clitic pronoun lui.

How would I say “She lends him one (of hers)” if she has several?

General “one (of them)”: Elle lui en prête un. (doesn’t say whose)
Specifically “one of hers”: Elle lui prête un des siens.

Any quick pronunciation tips for this sentence?

oublié: the final -é is [e]; liaison often smooths “a oublié.”
rasoir: final -r pronounced; oi ≈ [wa].
lui: one syllable [lɥi].
prête: open vowel [prɛt].
sien: nasal [sjɛ̃].
Say it smoothly: “Il a oublié son rasoir; elle lui prête le sien.”