La meva mare només vol una torrada amb mantega i un iogurt.

Because Catalan possessives agree with the thing being possessed, not with the speaker.

• mare is feminine singular
• so the possessive must also be feminine singular: meva

Compare:

• el meu pare = my father
• la meva mare = my mother
• els meus amics = my friends
• les meves germanes = my sisters

In standard Catalan, it is very normal to use the definite article before a possessive:

• la meva mare
• el meu pare
• els meus llibres

So although it looks more literal than English, la meva mare simply means my mother, not something extra like the mother of mine.

You may sometimes hear article omission in some special contexts or varieties, but for a learner, article + possessive + noun is the safe standard pattern.

Només means only or just.

So:

• La meva mare només vol... = My mother only wants...
• it suggests that this is all she wants, nothing more

It is a very common everyday word.

Because in this sentence it naturally modifies the whole verb phrase: only wants.

So:

• La meva mare només vol una torrada amb mantega i un iogurt

means that the only thing your mother wants is that combination of food.

Catalan word order is fairly flexible, but changing the position of només can slightly change the focus. For example:

• La meva mare vol només una torrada...

puts stronger focus on just one toast...

The original sentence is a very natural neutral order.

Vol is the 3rd person singular present tense of voler, which means to want.

A few present-tense forms are:

• jo vull = I want
• tu vols = you want
• ell / ella vol = he / she wants

So la meva mare vol means my mother wants.

Torrada usually means a slice of toasted bread, so in natural English it is often translated as toast.

That is why:

• una torrada = a piece/slice of toast

This is slightly different from English, where toast is often uncountable. Catalan often treats it as a countable item here.

Because torrada is a feminine singular noun.

So it takes the feminine singular indefinite article:

• una torrada

Compare:

• un iogurt = a yogurt
• una torrada = a toast / a slice of toast

Here mantega means butter in a general ingredient sense, so Catalan normally leaves out the article.

That is very common with food combinations:

• pa amb tomàquet
• cafè amb llet
• torrada amb mantega

If you meant some specific butter, then the article could appear, for example:

• amb la mantega que vam comprar ahir

But in the basic sentence, amb mantega is exactly what you would expect.

Because the indefinite article depends on gender, not on the first letter.

• un = masculine singular
• una = feminine singular

Iogurt is masculine, so it is:

• un iogurt

Catalan does not change un here just because the next word begins with a vowel.

Yes. I means and.

So the last part is:

• una torrada amb mantega i un iogurt

That means:

• one toast with butter
• and one yogurt

Also, amb mantega goes with torrada, not with iogurt. So it is not saying a yogurt with butter.

A simple English-friendly approximation is:

• només ≈ noh-MESS
• iogurt ≈ yoo-GOORT

A few helpful points:

• the accent in només shows the stress falls on the last syllable
• the initial i in iogurt sounds like English y in this word

Exact pronunciation varies a little by dialect, but those approximations are good enough for a learner.

Yes. Catalan also has alternatives such as:

• solament
• tan sols

But només is the most common and everyday choice.

So for normal conversation, només is the best word to learn first.