Majmun je sjedio na grani i gledao ljude.

Because this is the normal Croatian past tense structure, called the perfect.

It is formed with:

• the auxiliary je = is/has as a helper verb
• the past participle sjedio = sat

So je sjedio means was sitting / sat.

The same thing happens with gledao:

• full form: je gledao
• meaning: was watching / watched

In this sentence, the auxiliary je is used once and understood for both verbs:

• Majmun je sjedio ... i gledao ...
• literally: The monkey was sitting ... and watching ...

The endings match the gender and number of the subject.

The subject is majmun = monkey, which is:

• masculine
• singular

So the past participles are masculine singular:

• sjedio
• gledao

If the subject were feminine, you would get:

• Majmunica je sjedila i gledala.

If it were plural masculine:

• Majmuni su sjedili i gledali.

Because both verbs have the same subject and belong to the same past-time frame, Croatian often uses the auxiliary only once.

So:

• Majmun je sjedio na grani i gledao ljude.

means:

• Majmun je sjedio na grani i je gledao ljude — but this second version is not how you would normally say it.

In natural Croatian, one auxiliary can be shared across coordinated verbs.

Because na can take different cases depending on meaning.

Here the monkey is located on the branch, not moving onto it. So Croatian uses the locative after na for location.

• base noun: grana = branch
• locative singular: grani

So:

• na grani = on the branch

Compare:

• Majmun je na grani. = The monkey is on the branch.
• Majmun je skočio na granu. = The monkey jumped onto the branch.

In the second sentence, na granu is accusative because it shows movement toward a destination.

Because ljude is the accusative plural, and here people is the direct object of gledao.

The verb gledati takes a direct object:

• gledati koga/što = to watch whom/what

So:

• gledao ljude = watched people

This noun is a bit irregular:

• nominative plural: ljudi = people
• accusative plural: ljude

So:

• Ljudi stoje vani. = People are standing outside.
• Vidim ljude. = I see people.

The dictionary forms are:

• sjediti = to sit
• gledati = to watch / to look at

The sentence uses their past forms:

• sjedio from sjediti
• gledao from gledati

These are both very common verbs.

Because sjedio and sjeo do not mean the same thing.

• sjediti / sjedio = to be sitting / was sitting
• sjesti / sjeo = to sit down / sat down

So:

• Majmun je sjedio na grani means the monkey was already in that position.
• Majmun je sjeo na granu means the monkey moved into that position.

This is an example of aspect:

• sjediti is imperfective: ongoing state
• sjesti is perfective: completed action

In Croatian, gledati often means watch, look at, or observe, depending on context.

Here gledao ljude suggests the monkey was watching the people for some time, not just giving them one quick glance.

A quick or single glance could use a different verb, such as:

• pogledati = to look at / glance at

So:

• gledao ljude = ongoing watching
• pogledao ljude = looked at them once / briefly

I means and.

It joins the two actions done by the same subject:

• sjedio na grani
• gledao ljude

So the monkey was doing both things:

• sitting on the branch
• watching people

No, Croatian word order is fairly flexible, but this sentence uses a very natural, neutral order:

• Majmun je sjedio na grani i gledao ljude.

You can move things around for emphasis, for example:

• Na grani je majmun sjedio i gledao ljude.
• Ljude je majmun gledao sjedeći na grani.

But those versions sound more marked or stylistically different. For a learner, the original sentence is the best neutral model.

Yes. Croatian past tense does not map perfectly onto English past tense choices.

So:

• je sjedio can be translated as sat or was sitting
• gledao can be translated as watched or was watching

In this sentence, English would often prefer:

• The monkey was sitting on a branch and watching people.

But depending on style and context, sat and watched can also work.

Because Croatian has no articles.

So majmun can mean:

• a monkey
• the monkey

And grani can be:

• on a branch
• on the branch

And ljude can mean:

• people
• the people

English has to choose an article, but Croatian usually leaves that to context.