Las judías con arroz llenan bastante, pero mi hermano prefiere el maíz con atún.

In Spain, judías is a very common word for beans. A learner might know frijoles, but that is much more common in many Latin American countries than in Spain.

A few regional equivalents are:

• judías — common in Spain
• frijoles — common in much of Latin America
• habichuelas, porotos, etc. — used in other regions

So in Spanish from Spain, las judías sounds natural.

Judías is a feminine plural noun, so it takes the feminine plural definite article las.

• la judía = the bean
• las judías = the beans

Even though the ending -a often suggests feminine, it is always best to learn the noun together with its article when possible.

Because the subject is las judías con arroz, and the main noun there is judías, which is plural.

So the verb must agree with that plural subject:

• La judía llena = The bean is filling
• Las judías llenan = The beans are filling

The phrase con arroz does not change the number of the subject. The subject is still centered on judías.

Yes, literally llenar means to fill, but when talking about food, llenar often means to be filling or to make you feel full.

So:

• Las judías con arroz llenan bastante
  = Beans with rice are quite filling = literally, Beans with rice fill you quite a lot

This is a very natural way in Spanish to talk about food that satisfies your appetite.

Because in Spanish, when talking about food making you feel full, the usual verb is llenar, not ser lleno/a.

• Natural: Esto llena mucho = This is very filling
• Less natural here: Es muy lleno would not normally be used this way

So llenar is the standard choice for this meaning.

Here bastante means something like:

• quite
• fairly
• rather
• a lot, depending on context

In this sentence, it modifies the verb llenan, so it works as an adverb:

• llenan bastante = they are quite filling / they fill you up quite a bit

A useful point: when bastante is an adverb, it does not change form.

• come bastante = eats quite a lot
• llenan bastante = are quite filling

Con means with, and it often sounds more natural when two ingredients are presented as one combined dish.

• judías con arroz = beans with rice
• maíz con atún = corn with tuna

If you said judías y arroz, it could sound more like simply listing two separate foods: beans and rice. That is not wrong, but con often highlights the idea of a dish or mixture.

Spanish often uses the definite article with nouns when speaking about food or things in a general sense.

So:

• el maíz = corn
• el atún = tuna

In the sentence, el maíz con atún refers to that food combination as a noun phrase, so el is natural.

Spanish uses articles more often than English does in these contexts.

After con, Spanish often omits the article when talking about an ingredient in a general way.

So:

• maíz con atún = corn with tuna
• pasta con tomate = pasta with tomato
• pan con queso = bread with cheese

You could sometimes see an article in other contexts, but here no article is the normal choice.

Prefiere is from the verb preferir, meaning to prefer.

Here it is:

• mi hermano prefiere... = my brother prefers...

This is the third person singular present form, used for he, she, or it.

A quick pattern:

• yo prefiero = I prefer
• tú prefieres = you prefer
• él/ella prefiere = he/she prefers

Notice that preferir is a stem-changing verb:

• e → ie in many forms

Spanish word order is flexible, but subject + verb is the most straightforward and neutral order here.

• Mi hermano prefiere el maíz con atún = neutral, standard
• Prefiere mi hermano el maíz con atún = possible, but less neutral and more marked

So the sentence uses the most natural everyday order for a learner.

Pero means but and introduces a contrast.

The contrast is:

• Las judías con arroz llenan bastante
• pero mi hermano prefiere el maíz con atún

So the idea is: the beans with rice may be filling, but despite that, his brother prefers the corn with tuna.

Yes. Atún means tuna.

In food contexts, it often refers to tuna as an ingredient, especially canned tuna:

• ensalada con atún = salad with tuna
• pasta con atún = pasta with tuna

So here maíz con atún would normally be understood as corn with tuna as a food combination.

Only if the subject were singular.

In this sentence, the subject is plural:

• Las judías con arroz → plural
• so: llenan

If you changed the subject to a singular noun, then you would use llena:

• La comida llena bastante = The food is quite filling
• El arroz llena bastante = Rice is quite filling

It can be understood as talking about food in a fairly general or contextual way, which is very common in Spanish.

Spanish often uses the definite article with foods even when the meaning is not strictly these exact beans or that exact corn, but rather the food item or dish in general.

So:

• Las judías con arroz llenan bastante can mean Beans with rice are quite filling
• el maíz con atún can mean corn with tuna as a dish or food choice

The articles do not necessarily make it sound narrowly specific in the way English the often would.