Mi prima desayuna avena con leche y miel cuando hace ejercicio por la mañana.

In standard modern Spanish, you normally use a possessive adjective like mi, tu, su, etc. without an article before a family noun:

• mi prima
• mi hermano
• mi madre

So mi prima is the normal way to say my cousin.

Using la mi prima is not standard in normal modern Spanish. You may see article + possessive in some old-fashioned, poetic, or regional uses, but learners should stick with mi prima.

Because mi here is the possessive adjective meaning my.

Compare:

• mi prima = my cousin
• mí = me / myself after a preposition, as in para mí

So:

• mi prima = correct
• para mí = correct

The accent distinguishes two different words.

Desayuna is the third-person singular present tense of desayunar (to have breakfast / to eat breakfast).

Conjugation:

• yo desayuno
• tú desayunas
• él/ella/usted desayuna

Since mi prima is she, the verb is desayuna.

Spanish often uses the present tense to talk about:

1. habits
2. routines
3. things that usually happen

So Mi prima desayuna avena con leche y miel... sounds like a habitual action: this is what she does, generally or regularly.

English often does the same:

• My cousin eats oatmeal...
• My cousin has oatmeal...

If you wanted to emphasize the action happening right now, Spanish would more likely use:

• Mi prima está desayunando...

But here the simple present is the natural choice.

Because Spanish usually omits subject pronouns when the verb ending already shows who the subject is.

• Desayuna already tells you it is he/she/you formal
• Mi prima makes it even clearer that it is she

So Ella mi prima desayuna... would be wrong, and Ella desayuna... would only be used if you wanted extra emphasis or contrast.

Normal Spanish:

• Mi prima desayuna...

Spanish often leaves out the article when talking about food in a general, non-specific way, especially after verbs like eating or drinking.

So:

• desayuna avena
• con leche y miel

This is similar to English using uncountable or generic food nouns without the:

• She eats oatmeal
• with milk and honey

You could add an article in some contexts, but it would change the feel:

• desayuna la avena would usually sound like a specific oatmeal, not oatmeal in general
• con la leche y la miel would also sound more specific

Here, no article is the most natural choice.

Because avena here is being treated as a mass/generic food noun, like oatmeal in English.

When you are talking about what someone typically eats, Spanish often uses the noun without an article:

• Come pan
• Bebe leche
• Desayuna avena

Using una avena would usually sound unnatural here, because avena is not being counted as one unit in this sentence.

Because one con can introduce both nouns in the list.

• con leche y miel = with milk and honey

This is the normal and natural structure. Spanish, like English, does not need to repeat the preposition before every item unless you want special emphasis or clarity.

Compare:

• con leche y miel = normal
• con leche, con miel y con canela = possible, but more emphatic or stylistically marked

Because cuando here is a conjunction meaning when, joining two parts of the sentence:

• desayuna avena...
• cuando hace ejercicio...

No accent is used in this kind of statement.

The accented form cuándo is used in:

1. direct questions
2. indirect questions
3. some exclamatory uses

Examples:

• ¿Cuándo hace ejercicio?
• No sé cuándo hace ejercicio.

But in the sentence you gave, it is not a question, so cuando is correct.

Spanish often uses the expression hacer ejercicio, literally to do/make exercise, to mean to exercise.

So:

• hace ejercicio = she exercises / she works out

This is a very common idiomatic expression.

Spanish can also use verbs like:

• entrenar = to train
• hacer deporte = to do sport / play sports

But hacer ejercicio is one of the most common general ways to say to exercise.

Because hacer ejercicio is a fixed expression where ejercicio is often used as an uncountable/general activity noun.

So:

• hacer ejercicio = to exercise
• hace ejercicio = she exercises

If you said hace ejercicios, that would usually suggest specific exercises, like individual workout movements or drills.

Compare:

• Hace ejercicio todos los días = She exercises every day
• Hace ejercicios de piernas = She does leg exercises

In Spain, por la mañana is the standard way to say in the morning.

Also:

• por la tarde = in the afternoon/evening
• por la noche = at night / in the evening

So this phrase is extremely common and natural in Peninsular Spanish.

You may hear en la mañana in some varieties of Latin American Spanish, but for Spanish from Spain, learners should strongly prefer por la mañana.

It means something like when(ever) she exercises in the morning.

In this sentence, cuando suggests a habitual situation, not one single event. So the idea is:

• on mornings when she exercises, she has oatmeal with milk and honey for breakfast

The sentence does not necessarily mean she exercises every morning. It means that if/whenever the exercise happens in the morning, that is what she eats.

Yes. Spanish word order is flexible, although some versions sound more neutral than others.

The original:

• Mi prima desayuna avena con leche y miel cuando hace ejercicio por la mañana.

You could also say:

• Cuando hace ejercicio por la mañana, mi prima desayuna avena con leche y miel.

This version brings the time/condition part to the front and may sound slightly more focused on when this happens.

Both are correct. The original is perfectly natural.

It means to have breakfast / to eat breakfast.

English does have the verb to breakfast, but it sounds formal or old-fashioned to many learners. So the best natural translation of desayunar is usually:

• to have breakfast
• sometimes to eat breakfast

Examples:

• Desayuno a las siete = I have breakfast at seven
• ¿Qué desayunas? = What do you have for breakfast?

So in this sentence, desayuna is simply the normal verb for what she eats at breakfast time.