Alguien robó mi bicicleta.

The accent mark shows that the stress falls on the last syllable: ro-BÓ.

It also helps distinguish robó from robo:

• robó = he/she stole or someone stole (preterite, 3rd person singular)
• robo = I steal (present) or theft/robbery as a noun, depending on context

So in Alguien robó mi bicicleta, the accent is important because it tells you this is a past-tense verb.

Robó is the preterite, which is used for a completed action in the past: someone stole it, and the action is seen as finished.

Robaba is the imperfect, which would suggest something ongoing, repeated, or background information. On its own, Alguien robaba mi bicicleta would sound strange in most situations, almost like someone was stealing my bicycle in the middle of the action.

So:

• Alguien robó mi bicicleta = someone stole my bicycle
• Alguien robaba mi bicicleta = someone was stealing my bicycle / used to steal my bicycle

For a single completed event, robó is the normal choice.

Alguien means someone or somebody.

It is used when the speaker does not know who did the action, or does not want to say who it was. In this sentence, it means the thief is unknown or unspecified.

Examples:

• Alguien llamó a la puerta. = someone knocked on the door
• Alguien robó mi bicicleta. = someone stole my bicycle

It works very much like someone in English.

In Spanish, possessive adjectives like mi, tu, su, nuestro, etc. normally go directly before the noun, without an article.

So you say:

• mi bicicleta
• tu coche
• su casa

Not:

• la mi bicicleta ❌

English learners sometimes expect an article, but Spanish does not use one here.

Because mi already determines the noun. In Spanish, possessive adjectives usually replace the article.

Compare:

• la bicicleta = the bicycle
• mi bicicleta = my bicycle

You do not combine mi with la in this structure.

Yes, it is completely correct and natural.

However, in everyday Spanish from Spain, people also very often say:

• Me robaron la bicicleta.
• Me han robado la bicicleta.

These are also very natural, and in some contexts they may sound even more conversational.

The difference is mainly in structure:

• Alguien robó mi bicicleta = someone stole my bicycle
• Me robaron la bicicleta = they stole my bicycle / someone stole my bicycle
• Me han robado la bicicleta = someone has stolen my bicycle

In Spain, the present perfect (me han robado) is especially common for a recent event.

Spanish often uses an indirect object pronoun plus the definite article for possessions, especially when the owner is already clear.

So instead of saying:

• Alguien robó mi bicicleta

Spanish can also say:

• Me robaron la bicicleta

Literally, that is closer to They stole the bicycle from me, but in natural English the meaning is usually just Someone stole my bicycle.

This pattern is very common in Spanish:

• Me duele la cabeza. = my head hurts / I have a headache
• Me rompieron el móvil. = they broke my phone
• Le robaron el coche. = they stole his/her car

The verb form tells you it is third person singular, which matches alguien.

• alguien = someone
• robó = he/she/it stole

Together, they mean someone stole.

Even though English uses they more and more for unknown people, Spanish still treats alguien grammatically as singular.

The word order is:

• Alguien = subject
• robó = verb
• mi bicicleta = direct object

So it follows the common Spanish pattern:

Subject + Verb + Object

That said, Spanish word order is more flexible than English. For emphasis, you might hear variations in other contexts, but this neutral order is very standard.

A simple pronunciation guide is:

al-GYEN ro-BO mi bi-si-KLE-ta

A few useful notes:

• gui in alguien sounds like gye, with the u silent
• robó is stressed on the last syllable: ro-BÓ
• in most of Spain, the c before e in bicicleta is pronounced like the th in thin, so it sounds roughly like bi-thi-KLE-ta
• in Latin American pronunciation, that same c would usually sound like s

So in Spain, a rougher guide would be:

al-GYEN ro-BO mi bi-thi-KLE-ta

Yes, grammatically you could, but it is less natural in this situation.

Alguien is the usual word when the person is unknown:

• Alguien robó mi bicicleta. = natural

Una persona robó mi bicicleta literally means a person stole my bicycle, which is possible, but it sounds more explicit and less idiomatic in everyday speech.

Usually, alguien is the better choice here.