Learn Hardcore French: Leur voisine est une adulte célibataire qui vient souvent dîner avec eux. - Their neighbor is a single adult who often comes to have dinner with them.

Leur voisine = their neighbor (they share one neighbor)
- leur = “their” + singular noun
Leurs voisines = their neighbors (they have several neighbors)
- leurs = “their” + plural noun

Here there is only one neighbor, so you must use leur (singular) with the singular noun voisine.

Why does voisine end with -e? What is the difference between voisin and voisine?

French nouns for people often have a masculine and a feminine form:

un voisin = a (male) neighbor
une voisine = a (female) neighbor

The -e at the end usually marks the feminine form in writing.

In the sentence, the neighbor is female, so we use:

Leur voisine = their (female) neighbor

Why is it une adulte and not un adulte?

The noun adulte can be used for both men and women. Traditionally it was often treated as masculine (un adulte) for everyone, but modern usage frequently makes it feminine when referring to a woman:

un adulte = an adult man / an adult (gender-neutral, but grammatically masculine)
une adulte = an adult woman (grammatically feminine)

Since voisine tells us the neighbor is a woman, the speaker chooses une adulte to keep the gender consistent: she is a female neighbor and a female adult.

Why do we say est une adulte instead of just est adulte?

Both are possible, but they are slightly different:

Elle est adulte.
- adulte is an adjective = She is an adult / She is grown up
- Focus on her state or maturity.
Elle est une adulte.
- une adulte is a noun = She is an adult (person)
- Categorizes her as a member of a group: an adult (not a child/teen).

In your sentence, une adulte célibataire is a noun phrase (a single adult). It sounds a bit more like a description of her social category or status than just her level of maturity.

How does célibataire work? Does it change for masculine / feminine / plural?

Célibataire can be both:

An adjective = single, unmarried
A noun = a single person

Forms:

Masculine singular: célibataire
Feminine singular: célibataire (same spelling)
Masculine plural: célibataires
Feminine plural: célibataires

Examples:

un homme célibataire = a single man
une femme célibataire = a single woman
des hommes célibataires = single men
des femmes célibataires = single women

In your sentence, célibataire is an adjective modifying adulte: une adulte célibataire = a single adult (feminine).

Why is the order une adulte célibataire and not une célibataire adulte?

In une adulte célibataire, the main noun is adulte, and célibataire is an adjective that describes that adult:

une adulte (main noun)
célibataire (adjective: what kind of adult?)

Une célibataire adulte is also grammatically possible, but it slightly changes the focus:

Main noun: célibataire (a single person)
Adjective: adulte (who is adult)

So:

une adulte célibataire = an adult who happens to be single
une célibataire adulte = a single person who happens to be an adult

In most contexts, une adulte célibataire sounds more natural for “a single adult” as in your English translation.

Why is it qui vient and not qui viennent?

The relative pronoun qui refers back to the word voisine.

Leur voisine (singular)
qui vient (verb agrees with voisine)

So the subject of vient is leur voisine, which is singular → vient (3rd person singular).

If you had several neighbors, you would say:

Leurs voisines sont des adultes célibataires qui viennent souvent dîner avec eux.
- voisines (plural) → qui viennent (plural)

What exactly is qui doing in this sentence?

Qui is a relative pronoun that introduces a relative clause and stands for leur voisine.

Structure:

Leur voisine
est une adulte célibataire
qui vient souvent dîner avec eux.

You can think of it as:

Leur voisine […] vient souvent dîner avec eux.

Instead of repeating leur voisine, French uses qui:

qui vient souvent dîner avec eux = who often comes to have dinner with them
It links the extra information (she often comes to dinner) back to voisine.

Why is it vient souvent dîner and not vient à dîner or vient pour dîner?

French often uses the pattern:

venir + infinitive = to come (in order) to do something

So:

venir dîner = to come (in order) to have dinner

Adding souvent (often):

vient souvent dîner = comes often to have dinner / often comes to dinner

Using venir à dîner or venir pour dîner is either wrong or at least unnatural in this meaning. The simple, idiomatic structure is:

venir + infinitive (no preposition)

Why is souvent placed between vient and dîner?

When you have:

A conjugated verb + an infinitive
And an adverb like souvent

The usual order is:

[conjugated verb] + [adverb] + [infinitive]

So:

vient souvent dîner

Other positions are possible for emphasis (for example Souvent, elle vient dîner avec eux.), but the default neutral order inside this phrase is exactly what you see.

Is dîner here a verb or a noun? How can I tell?

Here dîner is a verb in the infinitive form.

Clues:

It comes directly after another verb (vient).
The pattern venir + infinitive is very common: venir manger, venir voir, venir aider, etc.

If dîner were a noun meaning “dinner”, you would usually see:

le dîner (with an article)
e.g. Elle vient pour le dîner. = She is coming for (the) dinner.

In your sentence, there is no article and it follows vient, so it is the infinitive verb: “to have dinner”.

Why is it avec eux and not avec ils or avec leur?

After prepositions like avec, French uses stressed pronouns (also called disjunctive pronouns), not subject pronouns.

Subject pronoun (used before verbs):

ils = they

Stressed pronoun (used after prepositions, for emphasis, etc.):

eux = them (masculine or mixed group)
elles = them (all-female group)

So you must say:

avec eux = with them
avec elles = with them (all women)

Leur is a possessive adjective or an indirect object pronoun (their / to them), so avec leur is incorrect here.

Could the sentence be Leur voisine vient souvent dîner avec eux. Elle est une adulte célibataire.? Is that different?

Yes, that is grammatically correct, but the structure and focus change:

Original sentence (with relative clause):
- Leur voisine est une adulte célibataire qui vient souvent dîner avec eux.
- Everything is packed into one sentence; “who often comes to have dinner with them” is attached directly to “a single adult”.
Two separate sentences:
- Leur voisine vient souvent dîner avec eux. Elle est une adulte célibataire.
- First you say what she does (often comes to dinner),
- Then as a separate piece of information you add that she is a single adult.

Meaning is very similar, but the original combines both facts into a single, smoother description.

How would the sentence change if the neighbor were a man?

You would have to make the relevant words masculine:

Leur voisin est un adulte célibataire qui vient souvent dîner avec eux.

Changes:

voisine → voisin (masculine neighbor)
une adulte → un adulte (masculine form)
célibataire stays the same in the singular
vient, souvent, dîner, avec eux all stay the same.