Mi madre compró otra docena de huevos y un kilo de patatas porque mañana vienen mis tíos a desayunar.

Compró is the preterite of comprar and means bought: a completed action in the past.

• mi madre compró = my mother bought
• mi madre compra = my mother buys / is buying
• mi madre ha comprado = my mother has bought

In this sentence, the buying is presented as a finished event, so compró is the most natural choice.

Also, the accent matters:

• compro = I buy / I bought depending on context? Actually in normal modern use, compro is I buy
• compró = he/she/it bought

So compró specifically tells you the subject is she here: mi madre.

The accent mark helps distinguish different forms of the verb.

• compro = I buy
• compró = he/she bought

Without the accent, the form would be read differently. In Spanish, accent marks are often important for telling verb forms apart.

Because docena is a feminine noun.

• una docena
• otra docena

Adjectives and determiners must agree in gender and number with the noun they modify:

• otro for masculine singular nouns
• otra for feminine singular nouns

So:

• otro kilo
• otra docena

It means another dozen eggs.

Breakdown:

• otra = another
• docena = dozen
• de huevos = of eggs

So literally: another dozen of eggs, but in natural English we say another dozen eggs.

The word otra suggests that some eggs had already been bought before, or that this is one more dozen in addition to what they already had.

In Spanish, quantities are commonly followed by de before the thing being measured or counted.

Examples:

• una docena de huevos = a dozen eggs
• un kilo de patatas = a kilo of potatoes
• un vaso de agua = a glass of water
• un poco de azúcar = a little sugar

So de is the normal connector after expressions of quantity, measure, or container.

Because the gender of the noun changes the article/adjective.

• kilo is masculine → un kilo
• docena is feminine → otra docena

So the words around the noun have to match it:

• un kilo
• otro kilo
• una docena
• otra docena

Because the quantity expression already determines the noun.

• otra docena de huevos
• un kilo de patatas

You do not normally say:

• una docena de los huevos
• un kilo de las patatas

unless you mean a very specific set already identified in context.

In this sentence, we are just talking about eggs and potatoes in a general shopping sense, so no article is needed.

In Spain, patatas is the usual word for potatoes. In many parts of Latin America, papas is more common.

So for Spanish from Spain:

• patatas is the most natural choice

A learner of Spain Spanish should definitely know patatas.

Porque (one word, no accent) means because.

• porque = because

Por qué (two words, with accent) is used in questions and means why.

• ¿Por qué compró más comida? = Why did she buy more food?

So in your sentence, the second part gives the reason, so porque is correct:

• ... porque mañana vienen mis tíos... = ... because tomorrow my tíos are coming...

Spanish often uses the present tense to talk about the near future, especially when the time is already clear from a word like mañana.

• mañana vienen mis tíos = my tíos are coming tomorrow

This is very natural Spanish. You could also say:

• mañana vendrán mis tíos

That is the simple future (will come), but in everyday speech the present tense is often more common for planned or expected future events.

Spanish word order is more flexible than English word order.

Both are possible:

• mis tíos vienen mañana
• mañana vienen mis tíos

Putting the subject after the verb is very normal in Spanish, especially when:

• the sentence introduces the action first
• the subject is not the main emphasis at the beginning
• the speaker wants a natural flowing rhythm

English usually prefers my uncles are coming tomorrow, but Spanish can comfortably say tomorrow are coming my uncles in structure, even though that exact order would sound odd in English.

Not always.

Tíos is the masculine plural of tío. In Spanish, the masculine plural can refer to:

• a group of males only = my uncles
• a mixed group = my aunts and uncles

So mis tíos could mean:

• my uncles
• my aunt and uncle
• my aunts and uncles

You understand the exact meaning from context.

After verbs of movement such as venir, ir, salir, Spanish often uses a + infinitive to express purpose.

• venir a desayunar = to come to have breakfast / to come for breakfast
• ir a comer = to go to eat
• salir a pasear = to go out for a walk

So here:

• mañana vienen mis tíos a desayunar

means they are coming in order to have breakfast.

It means to have breakfast.

English uses the expression have breakfast, while Spanish often uses a single verb:

• desayunar = to have breakfast
• comer = to eat / to have lunch in some contexts
• cenar = to have dinner

So vienen a desayunar is literally they come to breakfast, but natural English is they’re coming for breakfast or they’re coming to have breakfast.

Because mañana can mean either tomorrow or morning, and the context tells you which one is meant.

Here it means tomorrow because the sentence is explaining a future reason:

• porque mañana vienen mis tíos = because my tíos are coming tomorrow

If it meant the morning, you would more often see something like:

• por la mañana = in the morning

So:

• mañana = tomorrow
• por la mañana = in the morning

Yes. You could say:

• mañana van a venir mis tíos a desayunar

That means roughly my tíos are going to come tomorrow for breakfast.

All of these are possible:

• mañana vienen mis tíos...
• mañana van a venir mis tíos...
• mañana vendrán mis tíos...

They are all grammatical, but vienen sounds very natural for a planned near-future event in everyday speech.

Yes, usually a little.

• mi madre = my mother
• mi mamá / mamá = my mum / mom

In Spain, mi madre is very common and neutral. Mamá can sound more personal or familiar. The sentence uses mi madre simply as a neutral way to identify who did the shopping.