Learn Hardcore Spanish (Latin America): Mi hermana prepara una ensalada con lentejas y otros vegetales. - My sister prepares a salad with lentils and other vegetables.

Using la hermana by itself would mean “the sister”, which sounds incomplete in this context (the listener would ask: whose sister?). Spanish doesn’t allow you to drop the possessive here the way English sometimes can (e.g., “Sister made a salad” is impossible in Spanish).

Why is the verb prepara and not preparar or preparo?

Preparar is the infinitive (to prepare). You need to conjugate it to match mi hermana (she):

Yo preparo (I prepare)
Tú preparas (you prepare)
Él / Ella / Usted prepara (he / she / you-formal prepare)

Since mi hermana = ella (she), you use the third person singular: prepara.
So Mi hermana prepara… = “My sister prepares…” or “My sister is preparing…”, depending on context.

In English I would say “My sister is preparing a salad.” Why is it just prepara and not está preparando?

Spanish often uses the simple present (prepara) where English uses the present continuous (is preparing):

Mi hermana prepara una ensalada.
Can mean: “My sister prepares a salad” (habitually) or
“My sister is preparing a salad” (right now), depending on context.

You only need está preparando when you really want to emphasize that the action is happening right this moment:

Mi hermana está preparando una ensalada. = My sister is (right now) preparing a salad.

So prepara is more flexible and very common in everyday speech.

Why is it una ensalada and not la ensalada?

Una is the indefinite article (‘a / an’), and la is the definite article (‘the’).

una ensalada = a salad (not previously identified, not specific)
la ensalada = the salad (a specific one already known to speaker and listener)

In this sentence, we’re introducing the salad for the first time and talking about it in a non-specific way, so una is natural.
If you had already mentioned the salad, you might switch to la:

Mi hermana prepara una ensalada. La ensalada es muy grande.
My sister prepares a salad. The salad is very big.

How do I know that ensalada is feminine, and is that why it’s una?

Yes. Ensalada is a feminine noun, so its article must agree in gender and number:

una ensalada (feminine, singular)
las ensaladas (feminine, plural)

There isn’t always a rule that lets you predict gender perfectly, but many nouns ending in -a are feminine, and ensalada follows that pattern. The article/adjectives will always agree:

una ensalada deliciosa (fem. sing.)
dos ensaladas deliciosas (fem. pl.)

Why is it con lentejas and not de lentejas? What’s the difference?

Both con and de can relate ingredients to a dish, but the nuance is different:

con lentejas = with lentils (lentils are one of several ingredients)
de lentejas = of lentils / lentil (the main defining ingredient)

So:

ensalada con lentejas = a salad that includes lentils among other things
ensalada de lentejas = a lentil salad (lentils are the base or main feature)

In your sentence, con lentejas y otros vegetales highlights that lentils are just one ingredient among others, so con fits well.

Why is there no article before lentejas? Why not con las lentejas?

In Spanish, when you list ingredients in a general way, you usually omit the article:

ensalada con lentejas, tomate y cebolla
café con leche
pasta con salsa de tomate

Using las lentejas would sound like you’re referring to specific lentils already known in the conversation:

Mi hermana prepara una ensalada con las lentejas que compramos ayer.
My sister makes a salad with the lentils we bought yesterday.

In your sentence, it’s just “lentils” in general as an ingredient, so con lentejas (no article) is natural.

Why is lentejas plural? In English we often say “lentil salad” using “lentil” like an adjective.

In Spanish, lentejas are usually thought of as countable beans in the plural, even when you speak generally:

Me gustan las lentejas. = I like lentils.
Una sopa de lentejas. = Lentil soup.

We don’t use lenteja in the singular the way English uses “lentil” as a sort of adjective. The plural is the normal choice when they’re food on your plate or an ingredient you can count.

What’s the difference between vegetales and verduras? Which is more common in Latin America?

Both mean “vegetables”, but usage varies by region:

In much of Latin America, vegetales is very common and is understood everywhere.
Verduras is also used and understood, but in some places it can lean more toward “greens / leafy vegetables,” depending on context.
In Spain, verduras is more common in everyday speech, and vegetales can sound a bit more technical or scientific.

In your Latin American context, otros vegetales is perfectly natural and common.

Why is it otros vegetales, and how does otros agree with vegetales?

Otros is an adjective meaning “other” and must agree in gender and number with the noun it modifies:

otro vegetal (masculine singular)
otros vegetales (masculine plural)
otra verdura (feminine singular)
otras verduras (feminine plural)

Since vegetales is masculine plural, you use otros (masc. pl.).
If you said otras verduras, that would also be correct (both words feminine plural).

Could I change the word order, like Mi hermana prepara con lentejas y otros vegetales una ensalada?

Spanish word order is flexible, but not every order sounds natural. The most neutral and common order is:

Subject – Verb – Object – Extra information
Mi hermana (subject) prepara (verb) una ensalada (direct object) con lentejas y otros vegetales (extra info).

You could say Mi hermana prepara una ensalada de lentejas y otros vegetales, but moving con lentejas y otros vegetales between prepara and una ensalada (…prepara con lentejas y otros vegetales una ensalada) usually sounds awkward and marked.
In general, keep:

prepara una ensalada con lentejas…

Why don’t we say Ella mi hermana prepara… or Ella prepara…? Is dropping the subject pronoun normal?

Spanish is a pro-drop language: the subject pronoun (yo, tú, él, ella, etc.) is often omitted because the verb ending already shows who is doing the action.

Prepara already tells you “he/she/you-formal prepares.”
Mi hermana makes it fully clear who the subject is.

So you normally say:

Mi hermana prepara una ensalada…

You could say Ella prepara una ensalada…, but that just means “She makes a salad” without specifying that she’s your sister.
Saying Ella, mi hermana, prepara… is possible but has a different structure: “She, my sister, prepares…”, which is more like an apposition and not needed in this simple sentence.

Could I say Mi hermana hace una ensalada… instead of prepara? Is there a difference?

Yes, hace would be understood:

Mi hermana hace una ensalada… = My sister makes a salad…

However:

preparar often focuses on the process of getting ingredients ready, combining them, seasoning, etc.
hacer is more general: to make / to do.

In the context of cooking, both are widely used, but preparar una ensalada sounds slightly more specific to “prepare the dish”, while hacer una ensalada is more neutral and very common in conversation.