Learn Hardcore French: Je ne connais personne qui comprenne vraiment ma passion pour la grammaire. - I know nobody who really understands my passion for grammar.

Comprenne is in the subjunctive mood (subjonctif). It’s used here because the sentence expresses something non‑existent / indefinite under a negative:

Je ne connais personne qui comprenne…
= I don’t know anyone who (really) understands…

With ne… personne, you’re saying that such a person does not exist (for the speaker). In French, after expressions of non-existence, doubt, or uncertainty, a following relative clause with qui / que typically takes the subjunctive:

Je cherche quelqu’un qui comprenne.
Il n’y a personne qui puisse m’aider.

If you used the indicative (qui comprend), it would sound as if you actually have a specific person in mind who does exist and does understand—so it would contradict ne connais personne. Subjunctive keeps the idea “no such person (really) exists (for me)”.

How exactly does ne… personne work? Is it a “double negative”?

Ne… personne is a single negative structure, just split into two parts:

ne = the first part of the negation (often called the “ne” explétif in general negations)
personne = “no one / nobody”

Together: ne… personne = “nobody / no one”.

Basic pattern:

Je ne vois personne. = I don’t see anyone / I see nobody.
Il ne connaît personne ici. = He doesn’t know anyone here.

In English this looks like a “double negative” (not… no one), but in French it’s just one negation split into two pieces. French negative pairs:

ne… pas
ne… jamais
ne… plus
ne… rien
ne… personne
ne… aucun(e)

So no, in French it’s not incorrect or “double-negativish”; it’s simply how negation is formed.

Why is connais used here and not sais?

French connaître and savoir both translate as “to know”, but they’re used differently:

connaître = to be acquainted with, to know (be familiar with) people, places, things
- Je connais Marie. = I know Marie.
- Je connais ce livre. = I know this book / I’m familiar with it.
savoir = to know a fact / know how to do something
- Je sais où elle habite. = I know where she lives.
- Je sais parler français. = I know how to speak French.

In Je ne connais personne…, we’re talking about knowing people (being acquainted with people who understand your passion), so connaître is the only correct choice.

You cannot say Je ne sais personne; that’s simply wrong in French.

What does qui refer to here, and why is comprenne singular?

In Je ne connais personne qui comprenne vraiment ma passion pour la grammaire, the relative pronoun qui refers back to personne:

personne (antecedent) → qui (relative pronoun) → subject of comprenne

Even though personne is conceptually “no person / nobody” (which might feel like “no people”), grammatically it is singular in French. So the verb in the relative clause must also be singular:

personne qui comprenne (3rd person singular)
personne qui puisse m’aider
personne qui sache ce que je ressens

If you changed the antecedent to a plural, the verb would become plural:

Je connais des gens qui comprennent ma passion.
Ce sont les seuls qui connaissent vraiment ma passion.

Could I say Je ne connais personne qui comprend vraiment…? How wrong would that be?

Native speakers do sometimes use the indicative (comprend) in casual speech, but:

In standard / careful French, with ne… personne and a relative clause, the subjunctive (comprenne) is what’s expected.
Je ne connais personne qui comprend vraiment… may sound slightly off or less educated, because it clashes with the usual rule: non‑existence / indefinite antecedent → subjunctive.

So:

For exams, writing, or whenever you want correct, standard French:
→ use qui comprenne.
In everyday talk, you might hear qui comprend, but it’s safer not to copy that as a learner.

Why is vraiment placed before ma passion? Could I move it?

In qui comprenne vraiment ma passion pour la grammaire, vraiment is an adverb meaning “really / truly”. Here it modifies the verb comprenne:

comprenne vraiment = “(really) understands”

The usual neutral position is right after the verb in simple tenses:

Il comprend vraiment. = He really understands.
Ils respectent vraiment mon travail. = They really respect my work.

In this sentence, the object (ma passion pour la grammaire) is long, so vraiment naturally falls between the verb and its long object:

qui comprenne vraiment ma passion pour la grammaire.

You could also say:

qui comprenne ma passion pour la grammaire, vraiment. (with a pause; more emphatic, more spoken)
qui comprenne ma passion pour la grammaire vraiment. (possible, but the adverb then feels like it modifies the whole idea rather than directly the verb; less natural here)

The given placement (comprenne vraiment ma passion…) is the most natural in standard French.

Why is it pour la grammaire and not something like de la grammaire?

With passion, French normally uses the preposition pour to express what the passion is directed toward:

une passion pour la musique
une passion pour le cinéma
une passion pour la grammaire

So:

ma passion pour la grammaire = my passion for grammar

Using de would sound odd or would change the meaning:

ma passion de la grammaire → not idiomatic for “my passion for grammar”.
la passion de Marie pour la grammaire = Marie’s passion for grammar
(here de Marie just indicates whose passion it is.)

Also note the definite article la grammaire. French usually uses a definite article with abstract nouns in a general sense:

J’aime la musique. = I like music.
J’adore la poésie. = I love poetry.
J’ai une passion pour la grammaire. = I have a passion for grammar.

Is ne obligatory here? Can people say Je connais personne qui comprenne…?

In standard written French, ne is obligatory:

Je ne connais personne qui comprenne vraiment ma passion… ✅

In informal spoken French, many native speakers drop “ne” in everyday conversation:

Je connais personne qui comprenne vraiment ma passion… (spoken, informal)

So:

In writing, in exams, in formal contexts: always use ne.
In casual speech, you’ll very often hear Je connais personne…, Je vois personne…, etc.

As a learner, it’s best to learn and use the full form; later, you can recognize and possibly imitate the reduced form in speech.

Is there any nuance in ma passion pour la grammaire compared with just saying j’aime la grammaire?

Yes. Roughly:

J’aime la grammaire.
= I like grammar (maybe strongly, maybe moderately; neutral statement).
J’ai une passion pour la grammaire. / ma passion pour la grammaire
= I’m passionate about grammar; it’s more intense, stronger than simple liking.

So the original French sentence emphasizes that this is not just casual interest; it’s an intense, perhaps slightly niche, passion, and the speaker feels that no one really understands that intensity.