Deposité el sobre con la papeleta en la urna.

Because Spanish often drops subject pronouns when the verb already makes the subject clear.
Deposité tells you the subject is I.

So:

• Deposité el sobre... = I deposited the envelope...
• Yo deposité el sobre... is also possible, but it sounds more emphatic, as if you were stressing I.

Deposité is the 1st person singular preterite of depositar:

• yo deposité = I deposited
• tú depositaste
• él/ella depositó

The accent mark is important. It shows the correct stress and helps distinguish it from deposite (without an accent), which belongs to a different form set, such as the present subjunctive or the usted command.

Because the sentence describes a completed, single action: putting the envelope into the ballot box.

Spanish normally uses the preterite for that:

• deposité = I deposited / I put in

The imperfect (depositaba) would suggest something ongoing, repeated, backgrounded, or habitual, which does not fit this one-off action well.

Here sobre is a noun, meaning envelope.

You can tell from the article:

• el sobre = the envelope

When sobre is a preposition, it does not take an article in this way:

• un libro sobre la mesa = a book on the table
• hablar sobre política = to talk about politics

So in el sobre, it is clearly the noun envelope.

Spanish uses definite articles more often than English does. In this sentence, each noun refers to a specific, identifiable item in the voting situation:

• el sobre = the envelope
• la papeleta = the ballot paper
• la urna = the ballot box

Even where English might sometimes sound fine with fewer articles, Spanish often prefers them.

It means with the ballot paper inside it or containing the ballot paper.

So el sobre con la papeleta is:

• the envelope with the ballot paper

Grammatically, con la papeleta modifies el sobre. It describes the envelope.

It does not mean that the ballot paper is some kind of tool used to deposit the envelope. It is simply saying what the envelope contains.

Because depositar algo en un lugar is the normal pattern: you place something in/into something.

So:

• depositar algo en la urna = to deposit something in the ballot box

Using a would sound wrong here because a usually marks direction toward a person/place or the indirect object, while en is the natural choice for putting something into a container or enclosed place.

In voting language, urna means ballot box.

Yes, the literal/basic meaning is something like urn, but in electoral contexts it is the standard word for the box where votes are placed.

So in this sentence:

• la urna = the ballot box

Because urna is a feminine noun:

• la urna
• una urna

Learners sometimes wonder about this because some feminine nouns beginning with stressed a- use el in the singular, like el agua. But urna does not follow that pattern, so it stays:

• la urna electoral

Yes. In Spain, papeleta is the normal word for a ballot paper or voting slip.

In other Spanish-speaking countries, you may also hear words like boleta, depending on the region and the voting system. For a learner focusing on Spanish from Spain, papeleta is the expected word here.

It is natural, but it sounds a bit formal/neutral, which fits voting language very well.

In everyday speech, some people might also say things like:

• Metí el sobre en la urna
• Eché el sobre en la urna

But Deposité el sobre con la papeleta en la urna sounds especially appropriate in official, written, or careful narration.

Yes, that is grammatically possible. Spanish word order is fairly flexible.

Compare:

• Deposité el sobre con la papeleta en la urna.
• Deposité en la urna el sobre con la papeleta.

Both are correct. The first is the most neutral and straightforward. The second puts a little more focus on where the action happened.

Yes, but it would not mean exactly the same thing.

• Deposité la papeleta en la urna = I put the ballot paper in the ballot box.
• Deposité el sobre con la papeleta en la urna = I put the envelope containing the ballot paper in the ballot box.

The longer version is more precise because in some voting systems you cast an envelope containing the ballot paper, not the bare paper by itself.