Žirafa je bila blizu ograde i polako jela lišće.

Because je is a clitic in Croatian. Clitics are short unstressed words that usually go in the second position of the clause.

So:

• Žirafa je bila... = normal
• not usually Žirafa bila je...

This second-position rule is very common in Croatian and is something learners notice a lot.

This is the perfect tense, the most common past tense in everyday Croatian.

It is formed with:

• a form of biti = je
• plus the past participle = bila, jela

So:

• je bila = was
• (je) jela = ate / was eating

In Croatian, this one tense often covers both:

• English simple past: ate
• English past continuous: was eating

The exact English translation depends on context.

Because they agree with žirafa, which is feminine singular.

In Croatian, past participles agree with the subject in gender and number.

Here:

• žirafa = feminine singular
• so: bila, jela

Compare:

• masculine singular: bio, jeo
• feminine singular: bila, jela
• neuter singular: bilo, jelo

Croatian often leaves out a repeated auxiliary when two verbs are connected and share the same subject and tense.

So these are both possible:

• Žirafa je bila blizu ograde i polako jela lišće.
• Žirafa je bila blizu ograde i polako je jela lišće.

The version without the second je is very natural and common.

Because blizu normally takes the genitive case.

The base noun is:

• ograda = fence

Its genitive singular is:

• ograde

So:

• blizu ograde = near the fence

This is the same pattern as:

• blizu kuće = near the house
• blizu škole = near the school

Here blizu means near / close to.

In this sentence, it behaves like a word that introduces a noun phrase in the genitive:

• blizu ograde

Some grammars describe blizu as an adverb, some as a preposition-like word in uses like this. For a learner, the important practical point is:

• blizu
  • genitive

Here lišće is in the accusative singular, because it is the direct object of jela.

The giraffe was eating what?

• lišće

For this noun, the nominative and accusative forms look the same:

• nominative: lišće
• accusative: lišće

So the form does not change.

Lišće is grammatically singular neuter, but its meaning is collective. It means leaves / foliage in a general mass sense.

So:

• jela lišće = was eating leaves/foliage in general

If you wanted to emphasize individual leaves, you could use forms from list:

• list = leaf
• listove = leaves (accusative plural)

So there is a nuance:

• lišće = foliage, leaves as a mass
• listove = individual leaves

In this sentence, lišće sounds very natural.

Because Croatian has no articles like English a and the.

So žirafa can mean:

• a giraffe
• the giraffe

and ograde can mean:

• of a fence
• of the fence

The exact meaning comes from context, not from an article.

Yes, Croatian word order is fairly flexible, and polako can move around.

The sentence given is natural:

• Žirafa je bila blizu ograde i polako jela lišće.

But you could also hear:

• Žirafa je polako jela lišće.
• Žirafa je jela lišće polako.

The position in your sentence sounds smooth and natural because polako is placed right before the action jela.

Yes. Very often it can.

Croatian perfect tense does not always distinguish as sharply as English does between:

• ate
• was eating

So polako jela lišće could be understood as:

• ate leaves slowly
• was slowly eating leaves

In this sentence, because it describes a scene, English was slowly eating leaves is often the most natural translation.