Učiteljica je rekla da kvadrat nije isto što i krug, a da bez šestara nije lako nacrtati lijep krug.

Učiteljica means female teacher, while učitelj means male teacher.

Croatian often marks gender directly in nouns for people:

• učitelj = teacher (male)
• učiteljica = teacher (female)

The rest of the sentence agrees with that feminine subject, which is why you also get rekla and not rekao.

Je rekla is the usual Croatian perfect tense form, meaning said.

It is built from:

• je = is/has (auxiliary, 3rd person singular)
• rekla = said (past participle, feminine singular)

Because the subject is učiteljica (female teacher), the participle must be feminine:

• učitelj je rekao = the male teacher said
• učiteljica je rekla = the female teacher said

After verbs like reći (to say), Croatian very often uses da to introduce a clause equivalent to English that:

• Rekla je da... = She said that...

So:

• Učiteljica je rekla da kvadrat nije...
  means
• The teacher said that a square is not...

In everyday English, that is often omitted, but in Croatian da is very normal here.

The second da continues the reported statement.

The sentence is really saying:

• The teacher said that a square is not the same as a circle,
• and that without a compass it is not easy to draw a nice circle.

So Croatian repeats da:

• ... rekla da X, a da Y.

This is very natural. It keeps both clauses clearly dependent on rekla.

Here a links two ideas. It often means something like:

• and
• while
• but

In this sentence, the best natural sense is and or and also:

• ... da kvadrat nije isto što i krug, a da...
• ... that a square is not the same as a circle, and that...

It connects two related points rather than showing a strong contrast.

Nije isto što i means is not the same as.

Breakdown:

• nije = is not
• isto = the same
• što i = as

So:

• kvadrat nije isto što i krug
  = a square is not the same as a circle

This is a fixed and very common pattern:

• To nije isto što i ono. = That is not the same as that.
• A nije isto što i B. = A is not the same as B.

In Croatian, the negative form of je is nije.

So:

• je = is
• nije = is not

You do not normally say ne je.

Examples:

• On je učitelj. = He is a teacher.
• On nije učitelj. = He is not a teacher.

After the preposition bez (without), Croatian uses the genitive case.

The noun here is šestar = drawing compass.
Its genitive singular is šestara.

So:

• šestar = a compass
• bez šestara = without a compass

This is a very important pattern:

• bez vode = without water
• bez knjige = without a book
• bez šestara = without a compass

Yes, here šestar means a drawing compass: the instrument used to draw circles in geometry.

That matters because English compass can also mean the navigation tool. Croatian normally distinguishes them:

• šestar = drawing compass
• kompas = navigation compass

So in this sentence, bez šestara clearly means without a drawing compass.

This is an impersonal Croatian structure meaning it is not easy to draw.

Breakdown:

• nije lako = it is not easy
• nacrtati = to draw

Croatian often uses this pattern:

• Lako je razumjeti. = It is easy to understand.
• Nije teško naučiti. = It is not hard to learn.
• Nije lako nacrtati lijep krug. = It is not easy to draw a nice circle.

There is no explicit it in Croatian. English needs it, but Croatian does not.

Nacrtati is the perfective verb, meaning to draw / to produce a finished drawing.
Crtati is the imperfective verb, meaning to be drawing / to draw in a general or ongoing sense.

Here the sentence refers to achieving a result: drawing a nice circle successfully. That is why nacrtati fits well.

Compare:

• Volim crtati. = I like drawing.
• Mogu nacrtati krug. = I can draw a circle.
• Nije lako nacrtati lijep krug. = It is not easy to draw a nice circle.

Because krug is masculine singular, and here it is the direct object of nacrtati.

For masculine inanimate nouns, the accusative singular is the same as the nominative singular. So:

• lijep krug = a nice circle

You would expect lijepi in nominative plural or in some other forms, but not here.

Compare:

• To je lijep krug. = That is a nice circle.
• Želim nacrtati lijep krug. = I want to draw a nice circle.

Same form, because krug is masculine inanimate.

Because the sentence is making two separate points about circles:

1. A square is not the same as a circle.
2. Without a compass, it is not easy to draw a nice circle.

Croatian often repeats the noun instead of replacing it with a pronoun if that makes the sentence clearer. The repetition sounds natural here.

Croatian has no articles like a and the, so kvadrat can mean:

• a square
• the square

The exact meaning depends on context.

In this sentence, English would usually translate it as a square, because the sentence is speaking generally:

• a square is not the same as a circle

The same applies to krug:

• krug can mean a circle or the circle, depending on context.

Not completely. Croatian word order is fairly flexible, but this version sounds natural and clear.

This sentence puts the pieces in a standard order:

• Učiteljica je rekla...
• da kvadrat nije...
• a da bez šestara nije lako nacrtati...

You could move some parts for emphasis, but not every version would sound equally natural. For example, bez šestara could be moved, but the current placement neatly emphasizes without a compass before nije lako nacrtati.

In this context, lijep krug usually means something like:

• a nice circle
• a neat circle
• a well-shaped circle

Although lijep literally often means beautiful, here it is used in a practical sense: a circle that looks good, even, and properly drawn.