Avant de signer, je veux savoir si la propriétaire accepte mon garant.

Because after avant when it means before doing something, French normally uses de + infinitive.

• avant de signer = before signing
• avant de partir = before leaving
• avant de manger = before eating

So avant signer is not correct in standard French.

Because avant de is followed by an infinitive when the subject is understood in a general way or is the same as the main clause.

Here, signer means to sign. French is expressing the action in a non-conjugated form:

• Avant de signer, je veux savoir... = Before signing, I want to know...

If French uses a full clause with its own subject, it often uses avant que instead:

• Avant qu’il signe... = Before he signs...

Here si means whether / if, not if in the sense of a condition.

• je veux savoir si... = I want to know whether...

So this is an indirect yes/no question:

• Est-ce que la propriétaire accepte mon garant ?
• Je veux savoir si la propriétaire accepte mon garant.

Do not confuse this with conditional if:

• Si tu viens, je pars. = If you come, I leave.

Same word, different function.

Yes. Propriétaire means owner / landlord / landlady, and the article shows the gender here:

• le propriétaire = a male owner / landlord
• la propriétaire = a female owner / landlady

The noun often keeps the same written form, but the article changes. In speech, the feminine form may also be pronounced slightly differently depending on the speaker.

In housing and rental contexts, un garant is a guarantor: a person who promises to pay the rent if the tenant cannot.

This is a very common word in French rental situations.

Related words:

• un garant = a male guarantor
• une garante = a female guarantor

In everyday rental French, garant is the term you will see very often.

Because garant is a masculine noun, so the possessive adjective is masculine singular:

• mon garant
• ton garant
• son garant

If the noun were feminine, you would normally use ma:

• ma garante

So the possessive agrees with the noun being possessed, not with the speaker.

Because French often uses the present tense after expressions like je veux savoir si... when talking about a current situation or decision.

• je veux savoir si la propriétaire accepte mon garant = I want to know whether the landlady accepts my guarantor

This is about what her policy or decision is now. The present tense is the natural choice.

Because in French, the infinitive savoir comes directly after vouloir:

• je veux savoir = I want to know

Then the clause introduced by si comes after savoir:

• je veux savoir si...

French word order is much more fixed here than English learners sometimes expect.

Yes, absolutely. That would be more explicit.

• Avant de signer = Before signing
• Avant de signer le bail = Before signing the lease

French often leaves out words that are obvious from context, so signer by itself is natural if everyone already knows you are talking about the lease or contract.

No. It is an introductory phrase, not an independent sentence.

The main clause is:

• je veux savoir si la propriétaire accepte mon garant

And avant de signer gives the time relationship:

• before signing

So structurally, it works like an adverbial phrase at the beginning of the sentence.

Yes. The exact English translation depends on context.

In a rental context, la propriétaire often means:

• the landlady or
• the female owner

French uses the same word where English may choose a more specific one depending on the situation.

Because French normally does not use an article before a possessive adjective.

So you say:

• mon garant
• ma voiture
• mes amis

Not:

• le mon garant ❌

This is similar to English, where you say my guarantor, not the my guarantor.

Not in this sentence as written.

• Est-ce que is used for a direct question:

  • Est-ce que la propriétaire accepte mon garant ?

• Si is used for an indirect question:

  • Je veux savoir si la propriétaire accepte mon garant.

So once the question is embedded after je veux savoir, si is the correct choice.