Learn Hardcore French: Marie laisse entrer son ami dans la maison.

The structure laisser + infinitive means “to let/allow someone to do something.” In Marie laisse entrer son ami, laisser is conjugated (elle laisse) and entrer is the action she allows.

Why is there no preposition between laisser and entrer?

Because laisser is a catenative verb that directly takes an infinitive as its complement. You say laisser entrer quelqu’un without adding a preposition.

What role does son ami play in the sentence?

Son ami is the direct object of laisser (the person being allowed in) and simultaneously the logical subject of the infinitive entrer.

Why is the possessive adjective son used instead of sa?

Son is the third-person singular possessive for “his/her.” It’s used here because ami is masculine. (If the friend were feminine – amie – you would still say son amie to avoid the awkward vowel clash.)

What’s the difference between dans la maison and à la maison in this context?

Dans la maison specifies “inside the house” (emphasizing entry into its interior). À la maison simply means “at home” or “at the house,” without focusing on the action of entering.

Can the word order change to Marie laisse son ami entrer dans la maison? Does that alter the meaning?

Yes. Both Marie laisse entrer son ami dans la maison and Marie laisse son ami entrer dans la maison are correct and mean the same thing. Placing son ami before entrer is often more natural.

How would you put this sentence into the past tense (passé composé)?

You’d say: Marie a laissé entrer son ami dans la maison.
Here, laissé is the past participle of laisser, and you keep entrer in the infinitive.

How do you form the negative?

You insert ne … pas around the conjugated verb:
Marie ne laisse pas entrer son ami dans la maison.
Or you can also say Marie ne laisse pas son ami entrer dans la maison.