Moj brat danas ide na posao.

Because moj has to agree with brat in gender, number, and case.

• brat is masculine singular
• it is the subject of the sentence, so it is in the nominative
• therefore the correct form is moj

Compare:

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

So moj is the masculine singular nominative form.

Because Serbian does not use articles like English does.

English says:

• my brother
• the brother
• a brother

Serbian does not have separate words for the or a/an. The meaning is understood from context.

So:

• brat can mean brother, a brother, or the brother, depending on context
• moj brat naturally means my brother

Ide is the 3rd person singular present tense form of the verb ići, which means to go.

So:

• ja idem = I go / I am going
• ti ideš = you go / you are going
• on/ona/ono ide = he/she/it goes / is going

In this sentence, ide matches moj brat because my brother is he, singular.

Depending on context, ide can mean:

• goes
• is going

With danas (today), it often feels natural in English as is going today.

Because Serbian often uses the present tense for actions happening now, today, or in the near future, especially when the time is clear from context.

Here, danas already tells you when the action happens, so ide works perfectly.

This is similar to English sentences like:

• My brother is going to work today
• My brother goes to work today

In Serbian, the present tense can cover that idea naturally.

Because ići na posao is the normal Serbian expression for to go to work.

This is something learners usually have to memorize as a set phrase:

• ići na posao = to go to work
• biti na poslu = to be at work

Even though English uses to work, Serbian uses na here.

Very roughly:

• na often appears with destinations connected to activities, institutions, events, or functional places
• u is often used for going into an enclosed place

But with posao, the idiomatic expression is na posao.

In this sentence, posao is in the accusative singular because na is used with a verb of movement: ide na posao.

A very useful pattern is:

• na + accusative = motion toward something
• na + locative = location at/on something

Examples:

• Ide na posao. = He is going to work.
• On je na poslu. = He is at work.

Here:

• dictionary form: posao
• accusative singular: posao

So the form looks the same, but the case is accusative because of the movement.

Because some masculine nouns in Serbian have the same form in nominative and accusative, especially when they are inanimate.

Posao is an inanimate masculine noun, so:

• nominative: posao
• accusative: posao

That is why you do not see a visible ending change here.

Compare that with an animate masculine noun like brat:

• nominative: brat
• accusative: brata

So the case may change the form, but not always.

Yes. Serbian word order is more flexible than English word order.

The basic version here is:

• Moj brat danas ide na posao.

But you could also say:

• Danas moj brat ide na posao.
• Moj brat ide danas na posao.
• Na posao moj brat danas ide.
  This one sounds more marked or emphatic.

The meaning stays similar, but the focus changes.

For example:

• Danas at the beginning puts more emphasis on today
• Moj brat at the beginning puts more emphasis on my brother

So the original sentence is neutral and natural, but not the only possible order.

Sometimes yes, if the context already makes it clear whose brother you mean.

For example, in conversation, Serbian often leaves out words that are understood from context. But if you want to say my brother clearly and explicitly, moj brat is the normal form.

You may also hear structures like:

• Brat mi danas ide na posao.

Here mi means to me / my, so this also means my brother is going to work today.

That structure is very common in spoken Serbian, but for learners, moj brat is the easiest and clearest version to start with.

Because Serbian often drops subject pronouns when they are not needed.

The verb form already tells you the person and number:

• ide = he/she/it goes

So Serbian does not need to say on (he) unless there is a reason to emphasize or contrast it.

For example:

• Moj brat danas ide na posao. = My brother is going to work today.
• On danas ide na posao. = He is going to work today.

If the subject is already stated as moj brat, adding on would usually be unnecessary.

No. Danas is flexible.

You can place it in different parts of the sentence depending on what you want to emphasize:

• Moj brat danas ide na posao. = neutral
• Danas moj brat ide na posao. = emphasis on today
• Moj brat ide danas na posao. = also possible, slightly different rhythm

So danas is not fixed in one position. Serbian allows movement of adverbs much more freely than English does.

It can be understood either way, depending on context, but with danas it usually sounds like a specific event today.

Without danas, the sentence:

• Moj brat ide na posao.

could mean something general like My brother goes to work or My brother is going to work.

With danas, the sentence usually points to what is happening today:

• My brother is going to work today

So the time word helps narrow the meaning.

The dictionary form is ići = to go.

From this sentence, a useful mini-pattern to learn is:

• ići na posao = to go to work

And the present tense forms are worth memorizing:

• idem
• ideš
• ide
• idemo
• idete
• idu

So this one sentence gives you both a common verb and a very common everyday expression.