Mi madre dice que las fresas con un poco de melón están muy buenas en verano.

After decir, Spanish normally uses que to introduce what someone says:

dice que... = says that...

In English, that is often optional, but in Spanish que is usually required here. So Mi madre dice las fresas... would not work.

Spanish often uses the definite article when talking about foods or things in a general way.

So las fresas can mean:

• strawberries in general, or
• the strawberries in this food combination

English often drops the article in this kind of sentence, but Spanish usually keeps it.

Un poco de is a very common expression meaning a little bit of or some.

Examples:

• un poco de agua = a little water
• un poco de azúcar = a little sugar
• un poco de melón = a little melon

It works as a quantity expression, followed by de + noun.

Because here melón is being treated as an amount or ingredient, not as several whole melons.

After un poco de, Spanish often uses the singular when the noun is understood as a substance or food in an unspecified amount:

• un poco de pan
• un poco de queso
• un poco de melón

So the idea is a little melon, not a few melons.

Because the main subject is las fresas.

In las fresas con un poco de melón, the head noun is fresas, which is plural. The phrase con un poco de melón just adds extra information.

So:

• las fresas ... están
• not las fresas ... está

With food, estar bueno/a usually means to taste good or to be tasty.

So:

• Las fresas están muy buenas = the strawberries taste very good

If you use ser, the meaning changes. Ser bueno/a is more about being good in a general sense, good quality, or sometimes morally good depending on context. For how food tastes, estar is the natural choice.

Buenas agrees with fresas, which is feminine plural.

Agreement:

• la fresa está buena
• las fresas están buenas

It is not bueno, because that would be masculine singular. It is not bien, because bien is an adverb, and here Spanish uses an adjective with estar to describe the food.

It is possible in some contexts, but it does not sound as natural here.

• están muy buenas focuses on how they taste
• son muy buenas sounds more like a general statement about their quality or about strawberries as a type of fruit

In a sentence about eating fruit in summer, están muy buenas is the usual choice.

When speaking generally about seasons, Spanish often says:

• en verano
• en invierno
• en primavera

So en verano means in summer in a general sense.

If you are talking about a specific summer, you are more likely to use the article:

• en el verano de 2024

Con means with, but it does not tell you exactly how the fruit is served.

It could mean:

• mixed together
• served together
• eaten together

So the sentence gives the idea of strawberries with a little melon as a combination, without specifying the exact presentation.

Yes.

Both are correct, but they feel slightly different:

• mi madre = more neutral, a bit more formal
• mi mamá = more familiar and affectionate

In everyday speech, many people would naturally say mi mamá dice que...

Yes. Spanish has some flexibility in word order.

For example, you could also say:

En verano, mi madre dice que las fresas con un poco de melón están muy buenas.

That version puts the seasonal idea first. The original sentence is also completely natural; it just presents the information in a more neutral order.