On nije gladan, jer sada jede ručak.

Gladan agrees with the subject on (he), so it must be masculine singular.

Here is the pattern:

• on je gladan = he is hungry
• ona je gladna = she is hungry
• ono je gladno = it is hungry
• oni su gladni = they are hungry (masculine/mixed group)
• one su gladne = they are hungry (all feminine)

So in On nije gladan, the adjective has to match on.

In Serbian, the verb je (is) has an irregular negative form: nije.

So:

• on je gladan = he is hungry
• on nije gladan = he is not hungry

You do not normally say ne je here.

The same kind of thing happens in the plural:

• oni su gladni = they are hungry
• oni nisu gladni = they are not hungry

So nije and nisu are forms you simply need to learn as standard negative forms of to be.

Yes. Gladan is an adjective, and Serbian expresses this idea the same basic way English does:

• On je gladan. = He is hungry.

So the structure is:

subject + form of biti (to be) + adjective

Other similar examples:

• On je umoran. = He is tired.
• Ona je srećna. = She is happy.
• Dete je malo. = The child is small.

Jede is the 3rd person singular present tense of jesti (to eat), matching on.

Conjugation of jesti in the present:

• ja jedem = I eat / am eating
• ti jedeš = you eat / are eating
• on/ona/ono jede = he/she/it eats / is eating
• mi jedemo
• vi jedete
• oni/one/ona jedu

So sada jede means is eating now.

Because Serbian present tense often covers both:

• he eats
• he is eating

The word sada (now) makes it clear that this is an action happening right now, so in English we translate it as is eating.

Compare:

• On jede ručak. = He eats lunch / He is eating lunch.
• On sada jede ručak. = He is eating lunch now.

So the tense is present in Serbian, but the context tells you whether English should use simple present or present continuous.

Ručak is the direct object of jede, so it is in the accusative case.

However, for many masculine inanimate nouns, the accusative singular looks the same as the nominative singular. That is what happens here:

• nominative: ručak
• accusative: ručak

So the case is accusative, even though the form does not change.

Compare with another masculine inanimate noun:

• On čita roman. = He is reading a novel.

Again, roman is accusative, but it looks the same as nominative.

Serbian has no articles, so there is no direct equivalent of a/an/the.

That means ručak can mean:

• lunch
• the lunch
• sometimes a lunch

The exact meaning depends on context.

In this sentence, English naturally says lunch, without an article anyway: He is eating lunch now.

This is normal in Serbian:

• čitam knjigu = I am reading a/the book
• vidim psa = I see a/the dog

Context tells you which one is meant.

Jer means because and introduces a reason:

• On nije gladan, jer sada jede ručak.
• He is not hungry, because he is eating lunch now.

A very common alternative is zato što:

• On nije gladan zato što sada jede ručak.

Both are natural. Jer is short and very common in everyday Serbian.

In standard Serbian punctuation, a clause introduced by jer is usually separated by a comma.

So:

• On nije gladan, jer sada jede ručak.

This is standard written usage.

In very informal writing, people may be less careful with commas, but in correct writing the comma is expected here.

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

The sentence given is perfectly normal:

• On nije gladan, jer sada jede ručak.

But you may also hear or see:

• On sada jede ručak, pa nije gladan.
• Jer sada jede ručak, on nije gladan.
• Nije gladan, jer sada jede ručak.

The exact word order can change for focus, style, or emphasis.
Still, the original version is a very natural neutral sentence.

Sometimes yes, but in this sentence on is useful.

Serbian often drops subject pronouns when they are clear from the verb:

• Jedem. = I am eating.
• Radimo. = We are working.

But here nije gladan does not clearly identify the subject the way a strongly marked verb ending would. Also, gladan shows masculine singular, so on helps make the sentence fully clear and natural.

You could say:

• Nije gladan, jer sada jede ručak.

This can work if the context already makes it obvious who he is.

Because the sentence describes an action in progress: he is eating now.

For ongoing actions, Serbian normally uses the imperfective verb jesti.

• On sada jede ručak. = He is eating lunch now.

The perfective pojesti is used more for a completed action:

• Pojeo je ručak. = He ate / has eaten the lunch.
• Poješće ručak. = He will eat the lunch / will finish eating the lunch.

So jede is the right choice when the action is happening at this moment.

No. Sada and sad are both common.

• sada = full form
• sad = shorter, very common in speech

So you could also hear:

• On nije gladan, jer sad jede ručak.

Both are natural. Sada may sound a little more neutral or slightly more careful in style, while sad is very common in everyday conversation.