Moja sestra danas nosi haljinu, a ja nosim pantalone.

Because moj/moja/moje works like an adjective in Serbian, so it must agree with the noun it describes.

• sestra is feminine singular
• so my sister is moja sestra

Compare:

• moj brat = my brother
• moja sestra = my sister
• moje dete = my child

So moja is the feminine singular form needed for sestra.

Sestra is in the nominative singular, because it is the subject of the clause:

• Moja sestra danas nosi haljinu
• My sister is the person doing the action

So the basic dictionary form sestra is used here.

Because haljina is the direct object of the verb nosi.

The verb nositi often takes the accusative case for the thing being worn or carried. So:

• haljina = dress, nominative
• haljinu = dress, accusative singular

That is why Serbian says:

• nosi haljinu = is wearing a dress

This is a very common pattern:

• imam knjigu = I have a book
• vidim kuću = I see a house
• nosi haljinu = she is wearing a dress

Because pantalone is a plural noun, and in this sentence it is an inanimate direct object, so its accusative plural looks the same as its nominative plural.

So:

• pantalone = trousers/pants
• accusative plural: also pantalone

That is why you get:

• nosim pantalone = I am wearing pants

Also, pantalone is one of those nouns that is normally used only in the plural, much like English pants or trousers.

They are both present-tense forms of the verb nositi.

• nosi = he/she/it wears, carries
• nosim = I wear, carry

So in the sentence:

• moja sestra danas nosi haljinu = my sister is wearing a dress today
• ja nosim pantalone = I am wearing pants

This is just normal verb conjugation.

The basic verb is nositi.

It can mean:

• to wear clothing
• to carry something

In this sentence, because the objects are clothes (haljinu, pantalone), it clearly means to wear.

So:

• nosim kaput = I am wearing a coat
• nosim torbu = I am carrying a bag

The same Serbian verb can cover both ideas.

Yes, Serbian often leaves subject pronouns out, because the verb ending already shows the person.

For example:

• nosim pantalone already means I am wearing pants

But here ja is included for contrast:

• Moja sestra ... a ja ...
• My sister ..., and I ...

So ja is not strictly necessary for grammar, but it sounds natural because the speaker is contrasting my sister with me.

In this sentence, a introduces a contrast or comparison between two clauses.

• Moja sestra danas nosi haljinu, a ja nosim pantalone.

Here a is like:

• while
• whereas
• and with a contrastive feeling

So the idea is:

• My sister is wearing a dress today, while I’m wearing pants.

Why not the others?

• i = simple and, just adding information
• ali = but, usually stronger contradiction

So a is the most natural choice when you want a mild contrast between two parallel statements.

Because a is joining two independent clauses:

• Moja sestra danas nosi haljinu
• ja nosim pantalone

In Serbian, it is normal to put a comma before a when it connects two full clauses like this.

Danas means today. It is an adverb of time.

It tells us when the action happens:

• Moja sestra danas nosi haljinu = My sister is wearing a dress today

Yes, Serbian word order is fairly flexible, so danas can move around, although some positions sound more natural than others.

For example:

• Moja sestra danas nosi haljinu
• Danas moja sestra nosi haljinu
• Moja sestra nosi danas haljinu — possible, but less neutral in many contexts

The original version sounds natural and neutral.

Because Serbian does not have articles like English a, an, and the.

So:

• sestra can mean sister / the sister
• haljinu can mean a dress / the dress
• pantalone can mean pants / the pants

The exact meaning depends on context.

That is very normal in Serbian, and learners have to get used to understanding definiteness from the situation rather than from articles.

It can be either, depending on context.

Serbian present tense often covers both:

• she wears
• she is wearing

In this sentence, because of danas (today), the most natural English translation is usually:

• My sister is wearing a dress today, and I’m wearing pants.

But grammatically, Serbian is simply using the present tense.

Yes. You could say:

• Sestra danas nosi haljinu...

But then it would usually mean the sister / sister in context, not explicitly my sister.

Using moja sestra makes the relationship clear. Serbian often includes possessives when they are important or helpful, but sometimes leaves them out if the context already makes them obvious.

If you wanted a simpler and instead of a contrast, you could use i:

• Moja sestra danas nosi haljinu i ja nosim pantalone.

But this sounds less natural than the original if the point is to compare the two outfits.

The original with a is better because it naturally sets up a contrast:

• my sister is wearing a dress, while I’m wearing pants

So a is doing important work stylistically, not just grammatically.

Yes, pantalone is a standard and very common Serbian word for pants / trousers.

Depending on region and style, learners may also encounter other clothing words, but pantalone is perfectly normal and widely understood.

It behaves as a plural noun:

• ove pantalone = these pants
• nosim pantalone = I’m wearing pants

So it is a useful word to learn early.