Mi hermana usa otro buscador cuando el primero no encuentra nada útil.

In Spanish, you normally need a possessive like mi to say my sister.

• Mi hermana = my sister
• Hermana by itself usually means sister in a more general sense, not specifically my sister

So Mi hermana usa... is the natural way to say My sister uses....

Usa is the present tense of usar and means uses.

Here it describes a usual or repeated action:

• Mi hermana usa otro buscador... = My sister uses another search engine...

Spanish often uses the simple present for habits and general truths, just like English does:

• usa = uses
• está usando = is using

If you said está usando, it would sound like she is doing it right now, at this moment. But this sentence sounds more like a general habit or typical situation.

Also, utiliza is possible, but usa is more common and more everyday.

Otro means another or other.

In Spanish, otro normally does not use un before it:

• otro buscador = another search engine
• not un otro buscador

This is a standard rule:

• otro libro = another book
• otra idea = another idea

So usa otro buscador is exactly the natural structure.

Yes. Both can be used.

• buscador = search engine
• motor de búsqueda = search engine

Buscador is shorter and very common in everyday Spanish, including in Spain. In this sentence, it sounds natural and idiomatic.

So:

• Google es un buscador
• Google es un motor de búsqueda

Both are correct, but buscador is simpler and more conversational.

Cuando means when, and si means if.

Here, cuando suggests that this is what she does whenever that situation happens:

• when the first one doesn’t find anything useful

It sounds like a repeated or expected pattern.

Compare:

• cuando el primero no encuentra nada útil = when/whenever the first one doesn’t find anything useful
• si el primero no encuentra nada útil = if the first one doesn’t find anything useful

Both can work, but cuando often sounds more like a normal routine, while si focuses more on a condition.

Spanish often uses el primero to mean the first one when the noun is already understood from context.

So:

• otro buscador introduces the idea of search engines
• then el primero means the first search engine

This avoids repetition, just like English does:

• She uses another search engine when the first one...

You could say el primer buscador, but el primero is more natural here because the noun is already clear.

This is because primero is being used here as a pronoun, not directly before a noun.

Compare:

• el primer buscador = the first search engine
• el primero = the first one

Primer is the shortened form used before a masculine singular noun:

• el primer resultado
• el primer buscador

But when it stands alone, you use primero:

• el primero
• el segundo

Yes, and that is normal in Spanish.

Spanish commonly uses no together with words like nada, nadie, and nunca:

• no encuentra nada = doesn’t find anything
• literally: doesn’t find nothing

In English, that would usually be considered a double negative, but in Spanish it is the correct standard structure.

Examples:

• No veo nada = I don’t see anything
• No vino nadie = Nobody came / No one came
• No voy nunca = I never go

So no encuentra nada útil is exactly right.

Because the sentence is negative.

• nada útil = nothing useful / anything useful in a negative sentence
• algo útil = something useful

Compare:

• encuentra algo útil = he/she finds something useful
• no encuentra nada útil = he/she doesn’t find anything useful

After no, Spanish normally uses nada for anything/nothing in this kind of sentence.

Because útil is describing nada.

In Spanish, adjectives usually come after the noun or pronoun they describe:

• nada útil = nothing useful
• algo interesante = something interesting
• alguien famoso = someone famous

So útil follows nada in the normal Spanish order.

Because the subject is el primero, meaning the first one.

Even though primero refers to a search engine, it is still singular:

• el primero no encuentra = the first one doesn’t find

If it were plural, you would use encuentran:

• los primeros no encuentran

So the verb agrees with el primero, not with buscador in general.

Yes, but it is usually unnecessary.

Spanish often leaves out subject pronouns when the verb already makes the subject clear:

• Mi hermana usa...
• Mi hermana usa otro buscador cuando el primero no encuentra...

You could say:

• Mi hermana usa otro buscador cuando el primero no encuentra nada útil
• Mi hermana usa otro buscador cuando este no encuentra nada útil

But adding ella here would sound unnecessary unless you want emphasis or contrast:

• Ella usa otro buscador, pero yo no

Yes. Encontrar literally means to find, and it is very natural to use it with a search engine.

So:

• el buscador encuentra resultados = the search engine finds results
• no encuentra nada útil = it doesn’t find anything useful

Spanish uses encontrar in many of the same ways English uses find.

Yes, very strongly.

The whole sentence sounds like a typical repeated action:

• My sister uses another search engine whenever the first one doesn’t find anything useful

That comes from:

• the present tense: usa, encuentra
• the use of cuando, which often means whenever in this kind of context

If you wanted one specific past event, you would change the tense:

• Mi hermana usó otro buscador cuando el primero no encontró nada útil
  = My sister used another search engine when the first one didn’t find anything useful

Yes. The original sentence is already natural, but here are some possible alternatives:

• Mi hermana usa otro buscador cuando el primero no encuentra nada útil.
• Mi hermana usa otro buscador si el primero no encuentra nada útil.
• Mi hermana cambia de buscador cuando el primero no encuentra nada útil.

These have slightly different nuances:

• usa otro buscador = uses another search engine
• si = if, a bit more conditional
• cambia de buscador = switches search engine

But the original sentence is perfectly good standard Spanish.