En caso de que una palabra esté en femenino, mi hermana suele buscar primero su forma en masculino.

In standard Spanish, the usual structure is en caso de que + subjunctive.

• en caso de que = in case / in the event that
• It introduces a possibility or hypothetical situation, which is why it is normally followed by the subjunctive.

So:

• En caso de que una palabra esté... = In case a word is...

Using en caso que without de is not the standard form here.

Because it comes after en caso de que, which typically triggers the subjunctive when talking about something possible, uncertain, or hypothetical.

Here, the speaker is not saying that the word definitely is feminine every time, but rather talking about a possible case:

• está = indicative, for facts
• esté = subjunctive, for possibility/hypothesis in this structure

So:

• si una palabra está en femenino would sound more like a straightforward condition
• en caso de que una palabra esté en femenino sounds more formal and hypothetical: in case a word happens to be in the feminine

Esté is the present subjunctive of estar.

Here is the relevant form:

• yo esté
• tú estés
• él/ella/usted esté

In the sentence, una palabra is singular, so Spanish uses the él/ella form: esté.

Both ideas are possible in Spanish, but they are not exactly the same.

• ser femenina usually describes something as feminine in nature or quality
• estar en femenino means something is in the feminine form/gender

Here the sentence is talking about grammatical gender, not personality or style. So estar en femenino means that the word appears in its feminine grammatical form.

You may also hear:

• estar en masculino
• estar en plural
• estar en singular

These all refer to the form a word is currently in.

Because en femenino and en masculino are fixed grammatical expressions used to talk about the gender form of a word.

So:

• una palabra en femenino = a word in the feminine form
• su forma en masculino = its masculine form

If you said una palabra femenina, that could sometimes be understood differently, as a feminine word, which is less precise in a grammar explanation. En femenino focuses on the form or grammatical category.

Suele + infinitive means usually does or tends to do.

So:

• mi hermana suele buscar = my sister usually looks for / my sister tends to look for

The verb is soler, which is very commonly used for habits:

• Suelo leer por la noche. = I usually read at night.
• Suelen llegar tarde. = They tend to arrive late.

It is almost always used with an infinitive after it.

Both word orders are possible in Spanish, but buscar primero sounds very natural here.

• suele buscar primero su forma en masculino
• literally: usually looks first for its masculine form

Placing primero after the infinitive often sounds smoother and more neutral in everyday Spanish.

You could also say:

• mi hermana primero suele buscar...
• mi hermana suele primero buscar...

But those are less natural in this sentence.

Su refers to una palabra.

So:

• una palabra
• su forma en masculino = its masculine form

Even though su can mean his, her, its, your, their, the context makes it clear here that it means its.

Both can be possible, but forma en masculino is especially common when talking about grammar and dictionary forms.

• forma en masculino = form in the masculine
• forma masculina = masculine form

In practice, they are very close in meaning here. The version with en sounds a bit more explicitly grammatical, matching esté en femenino earlier in the sentence.

Yes, palabra is a feminine noun:

• la palabra
• una palabra

Its ending -a matches the very common pattern of feminine nouns, though Spanish does have exceptions. In this sentence, that is why you get:

• una palabra
• not un palabra

Spanish often drops subject pronouns like yo, tú, ella, because the verb ending already shows who is doing the action. But nouns like mi hermana are often included when the speaker wants to identify the person clearly.

Here, mi hermana is the subject of suele buscar.

You could not simply remove it unless the context already made it obvious who the speaker was talking about.

Compare:

• Mi hermana suele buscar... = My sister usually looks for...
• Suele buscar... = She usually looks for... / He usually looks for... / It usually looks for... depending on context

Yes, but the meaning and tone would change slightly.

• Si una palabra está en femenino... = If a word is in the feminine...
• En caso de que una palabra esté en femenino... = In case a word is in the feminine / In the event that a word is in the feminine...

Using en caso de que sounds more formal and a bit more hypothetical. Using si is often simpler and more direct.

Also notice the verb difference:

• si can take the indicative here: está
• en caso de que normally takes the subjunctive: esté

Yes, it is understandable and natural, especially in an explanatory or slightly careful register.

A native speaker might also say things like:

• Si una palabra aparece en femenino, mi hermana suele buscar primero la forma masculina.
• Cuando una palabra está en femenino, mi hermana suele buscar primero el masculino.

But the original sentence is perfectly valid, especially if the speaker wants to sound precise and structured.

Because in expressions like en femenino and en masculino, Spanish usually does not use the article.

These work like fixed grammatical labels:

• estar en femenino
• estar en masculino
• decirlo en plural

You would not normally say:

• en el femenino
• en el masculino

unless you were talking about something more specific in a different context.