Vuokrasopimuksessa lukee, että vuokravakuus maksetaan ennen kuin saan avaimet.

Vuokrasopimuksessa is the inessive case (ending -ssa/-ssä), meaning in the lease contract.

• vuokrasopimus = (a) lease contract (basic dictionary form, nominative)
• vuokrasopimuksessa = in the lease contract (where the text is written)

In Finnish, lukea can mean both to read and to say/to be written (somewhere). In this type of sentence, lukee is used like “it says”:

• Vuokrasopimuksessa lukee, että … = The lease says that … / It is written in the lease that …
  It’s a very common pattern, especially with contracts, signs, instructions, etc.

This is an impersonal / context-based usage. The “subject” is basically the text/contract, even if it’s not stated as a grammatical subject. Finnish often uses:

• X:ssä lukee, että … = “In X it says that …”
  No explicit “it” is needed the way English requires one.

että is the most common word for that when introducing a clause that reports content:

• lukee, että vuokravakuus maksetaan … = says that the deposit is paid …
  So everything after että is the content of what the lease states.

Because vuokravakuus is the subject of the clause vuokravakuus maksetaan … (even though the verb is passive). Finnish doesn’t have articles (a/the), so vuokravakuus can mean “the rent deposit” in context without any extra word.

Yes. vuokravakuus is the standard term for a rental/security deposit. It’s money the tenant pays to the landlord to cover potential damage/unpaid rent, typically returned at the end if everything is fine.

maksetaan is the passive present (“is paid”). Finnish commonly uses the passive when:

• the doer is obvious from context (here: the tenant), or
• the text is stating a general rule/policy (common in contracts)

So vuokravakuus maksetaan = the deposit is paid (i.e., “you/one pay(s) the deposit”).

Yes, but the meaning and tone change. Examples:

• Vuokravakuus maksetaan ennen kuin saan avaimet. (contract/policy style, neutral)
• Mak san vuokravakuuden ennen kuin saan avaimet. (personal, “I pay the deposit…”, more like a personal statement)

Note how in the active version, the object becomes vuokravakuuden (accusative/genitive-like form), not vuokravakuus.

ennen is used with nouns/phrases, while ennen kuin is used before a full clause with a verb.

• ennen avaimia / ennen avainten saamista = before the keys / before receiving the keys (noun/nominal structure)
• ennen kuin saan avaimet = before I get the keys (clause with saan)

saan is the 1st person singular present of saada = to get/receive.
So ennen kuin saan avaimet means before I receive/get the keys (i.e., before the landlord hands them over).

Finnish commonly uses the plural avaimet (“keys”) because you often get:

• multiple physical keys (door key, mailbox key, storage key), or
• a set of keys

Even if it’s literally one key, plural still sounds natural in this context.

avaimet is nominative plural, but as the object of saan it functions as a total object: you receive the whole set.
If you used avaimia (partitive plural), it would suggest an incomplete/unspecified amount, like “some keys” or “keys in general,” which doesn’t fit as well here.

You can rearrange parts, but some patterns are standard. Common alternatives:

• Vuokrasopimuksessa lukee, että vuokravakuus pitää maksaa ennen kuin saan avaimet. (adds pitää, “must”)
• Vuokravakuus maksetaan ennen avaimien saamista, näin vuokrasopimuksessa lukee. (more formal/stylized)

The given version is already very natural and contract-like.