Son rasoir est sous le miroir, mais elle préfère se raser plus tard.

Because rasoir is a masculine noun. In French, the possessive adjective (son/sa/ses) agrees with the gender and number of the noun possessed, not with the owner.

• Masculine singular: son rasoir
• Feminine singular: sa brosse
• Plural: ses affaires

Note: You also use son before a feminine noun starting with a vowel sound to avoid awkward pronunciation, e.g., son amie (not sa amie).

Son can mean “his,” “her,” or “its.” The owner is determined by context. Here, the next clause has elle, so the natural reading is “her razor.” If you need to remove ambiguity, you can say:

• Le rasoir de Marie est sous le miroir.
• Colloquially for emphasis: Son rasoir à elle est sous le miroir. (adds clarity but is informal)

The simple preposition sous (“under”) is followed directly by a determiner and noun: sous le/la/les…. So you say sous le miroir.
You use du (“de + le”) with expressions like en dessous du or au-dessous du, not after sous:

• sous le miroir = under the mirror (physically)
• en dessous du miroir / au-dessous du miroir = below the mirror (more about vertical position/level)

• sous + noun: physically underneath something. Example: Le chat est sous la table.
• dessous (adverb): “underneath/under there,” often without a noun. Example: Regarde dessous.
• en dessous de + noun: “below” (can be physical or figurative). Example: Il vit en dessous du seuil de pauvreté.
• au-dessous de + noun: similar to en dessous de, often more formal or for vertical scale. Example: La température est au-dessous de zéro.

For an object literally sitting under another, sous + noun is the most natural.

Raser means “to shave (something/someone).” When the subject shaves themself, French uses the reflexive form se raser (“to shave oneself”).

• Elle rase son frère. = She shaves her brother.
• Elle se rase. = She shaves (herself).

Object pronouns (including reflexive me/te/se/nous/vous) go right before the infinitive:

• Elle préfère se raser plus tard. Wrong: Elle se préfère raser.

With negation, the ne…pas wraps the conjugated verb, and the pronoun still sits before the infinitive:

• Elle ne veut pas se raser maintenant.

Yes. Se raser simply means “to shave,” regardless of gender. You’d specify the body part if needed:

• Elle se rase les jambes / les aisselles.
• Il se rase le visage.

Use the definite article with body parts after a reflexive verb, not the possessive:

• Correct: Elle se rase les jambes.
• Not idiomatic: Elle se rase ses jambes. The reflexive pronoun (se) already shows whose body part it is.

• plutôt = “rather/instead.” Example: Elle préfère partir plutôt que rester.
• plus tard = “later.” Example: Elle partira plus tard.

Don’t use plutôt to mean “later.”

Yes. In préférer, the second é changes to è (open e) when the syllable after it is silent in pronunciation:

• je préfère, tu préfères, il/elle préfère, ils/elles préfèrent
• But: nous préférons, vous préférez (no change because the ending is pronounced)

Both exist, but usage varies:

• miroir is the neutral, general word for a mirror in modern French.
• glace can also mean a mirror (especially in certain expressions or older usage), but it more commonly means “ice” or “ice cream” depending on context. You’ll still see it in compounds like armoire à glace (wardrobe with a mirror). For clarity, miroir is safest.