Mi vecina suele silbar cuando riega las plantas del balcón.

Mi vecina means my female neighbor.

• mi = my
• vecina = female neighbor

Spanish marks gender on the noun here:

• vecino = male neighbor
• vecina = female neighbor

Also, mi does not change for masculine/feminine singular nouns:

• mi vecino
• mi vecina

It only changes for number:

• mis vecinos
• mis vecinas

Suele silbar is the verb soler + infinitive, a very common structure in Spanish.

It means:

• usually whistles
• tends to whistle

So:

• Mi vecina suele silbar = My neighbor usually whistles

This is more natural in Spanish than always using an adverb like normalmente or a menudo, though those are possible too.

A key point: soler is almost always used with another verb in the infinitive:

• suelo leer = I usually read
• suelen salir = they usually go out

Riega comes from the verb regar, meaning to water.

This verb is a stem-changing verb in the present tense:

• regar
• e → ie in stressed forms

So:

• yo riego
• tú riegas
• él/ella riega
• nosotros regamos
• vosotros regáis
• ellos riegan

In the sentence, riega matches mi vecina, which is third person singular:

• Mi vecina riega...

Spanish often leaves out subject pronouns because the verb ending already tells you who the subject is.

So instead of saying:

• Mi vecina suele silbar cuando ella riega las plantas...

Spanish normally says:

• Mi vecina suele silbar cuando riega las plantas...

The subject of riega is understood to be the same person: mi vecina.

You could add ella, but it would usually sound emphatic or contrastive:

• ...cuando ella riega, no cuando otra persona lo hace.

Here cuando introduces a habitual/repeated action, so Spanish uses the present indicative:

• cuando riega = when she waters / whenever she waters

The sentence describes something that regularly happens:

• She usually whistles when she waters the plants

If you were talking about a future event that has not happened yet, Spanish would normally use the subjunctive:

• Silbará cuando riegue las plantas.
• She will whistle when she waters the plants.

So the difference is:

• cuando riega = habitual / general
• cuando riegue = future, not yet completed

Las plantas del balcón means the plants from/on the balcony or the balcony plants, depending on context.

Breakdown:

• las plantas = the plants
• del = de + el
• balcón = balcony

So literally:

• the plants of the balcony

In natural English, we would usually say:

• the plants on the balcony
• the balcony plants

Spanish often uses de where English prefers on, in, or an adjective-like structure.

Del is the contraction of de + el.

So:

• de el balcón → del balcón

This contraction is mandatory in normal Spanish.

Examples:

• la puerta del coche = the car door
• el color del cielo = the color of the sky

One important exception: there is no contraction if el is part of a proper name:

• de El Escorial (not del Escorial if the official name begins with El)

But in this sentence, it is just the normal article el, so del is required.

Spanish uses definite articles more often than English.

Here:

• las plantas = the plants
• el balcón = the balcony

This sounds natural because the speaker is referring to specific plants and a specific balcony associated with the neighbor.

English might sometimes say:

• when she waters her balcony plants

But Spanish often prefers the article:

• las plantas del balcón

So even when English uses a possessive or a different structure, Spanish may still use the.

In most of Spain, vecina is pronounced roughly like:

• beh-THEE-nah

More precisely:

• v and b sound very similar in Spanish
• c before i is usually pronounced like th in think in most of Spain

So:

• vecina ≈ beh-THEE-nah

In many parts of Latin America, it would sound more like:

• beh-SEE-nah

If you are learning Spanish from Spain, the th sound is the one you usually want.

The accent mark in balcón shows where the stress goes.

Without getting too technical, the written accent tells you to stress the last syllable:

• bal-CÓN

So it is:

• bal-KON

not:

• BAL-kon

Accent marks in Spanish are very important because they show pronunciation and sometimes distinguish words.

Yes. Spanish word order is flexible, especially with time clauses like cuando...

You could also say:

• Cuando riega las plantas del balcón, mi vecina suele silbar.

This means essentially the same thing:

• When she waters the balcony plants, my neighbor usually whistles.

The difference is mostly one of focus or style:

• Mi vecina suele silbar... starts with the person
• Cuando riega... starts with the situation/time

Both are natural.

Yes, sometimes.

You could say:

• Mi vecina silba cuando riega las plantas del balcón.

That can also mean:

• My neighbor whistles when she waters the balcony plants.

But suele silbar makes the idea of habit more explicit:

• she usually whistles
• she tends to whistle

So soler + infinitive adds a clearer sense of regularity.