Mon frère préfère le thon avec du maïs, mais ma sœur veut des crevettes avec des petits pois.

Because mon and ma are possessive adjectives, and they agree with the noun being possessed, not with the person who owns it.

• frère is masculine singular, so you use mon
• sœur is feminine singular, so you use ma

So:

• mon frère = my brother
• ma sœur = my sister

They are two completely different words:

• mais = but
• maïs = corn

The two dots over ï in maïs are very important. They show that the a and i are pronounced separately.

So:

• mais is one syllable
• maïs is two syllables

That is why the sentence has both mais and maïs, even though they look very similar.

In French, after verbs like aimer, adorer, détester, and préférer, you usually use the definite article when talking about things in general.

So French says:

• préférer le thon
• literally: to prefer the tuna
• natural English: to prefer tuna

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

Because here they mean some corn, some shrimp, and some peas, not those foods in a general sense.

There is a contrast in the sentence:

• préfère le thon = he prefers tuna in general
• veut des crevettes = she wants some shrimp
• avec du maïs = with some corn
• avec des petits pois = with some peas

So:

• le / la / les often gives a general meaning or refers to something specific
• du / de la / des often means some

Du is the partitive article for a masculine singular noun. With food, it often means some.

So:

• du maïs = some corn

Historically, du comes from de + le, but in sentences like this, you should think of it as the normal way to say some with a masculine singular food noun.

Other examples:

• du pain = some bread
• du riz = some rice
• du fromage = some cheese

Literally, yes: petits pois means small peas. But in everyday French, les petits pois is the normal expression for peas, especially green peas.

So in practice:

• des petits pois = some peas

It is a fixed expression, so learners should usually memorize it as a unit.

Both are in the present tense.

• préfère comes from préférer = to prefer
• veut comes from vouloir = to want

They are both third person singular forms:

• mon frère préfère = my brother prefers
• ma sœur veut = my sister wants

Roughly:

• frère ≈ frehr
• sœur ≈ a difficult French vowel, something a bit like sir or fur, but with rounded lips
• maïs ≈ mah-EES, with two clear syllables

A few pronunciation notes:

• The è in frère sounds like an open eh
• The œ in sœur is a special French vowel that does not exist exactly in English
• The ï in maïs forces you to pronounce the vowels separately: ma-is

Because each food item has its own accompaniment:

• le thon avec du maïs
• des crevettes avec des petits pois

Repeating avec makes the structure clear and natural. It shows that:

• the tuna comes with corn
• the shrimp come with peas

If you removed the second avec, the sentence would sound less clear.

Yes, often it can. In everyday French, crevettes is commonly translated as shrimp, but depending on the context or region, it may also overlap with prawns.

For most learners, shrimp is the safest basic translation.

Because mais means but, and it connects two contrasting ideas:

• Mon frère préfère le thon avec du maïs
• mais ma sœur veut des crevettes avec des petits pois

The comma helps separate the two parts, just like in English:

• My brother prefers tuna with corn, but my sister wants shrimp with peas.