Moj sin je nacrtao trokut, kvadrat i pravokutnik, ali je zaboravio povući zadnju crtu.

Because moj sin is the subject of the sentence, so it is in the nominative case.

• moj = my
• sin = son

So moj sin means my son as the person doing the action.

If son were the object instead, the form would change:

• Vidim svog sina. = I see my son.

Je nacrtao is the perfect tense in Croatian, used very often to talk about completed past actions.

It is made from:

• the auxiliary je = is/has (from biti, to be)
• the past participle nacrtao = drawn

So literally it is something like has drawn, but in normal English here you would translate it as drew.

Because sin is masculine singular, the participle is:

• nacrtao for masculine singular

Compare:

• Moj sin je nacrtao.
• Moja kći je nacrtala.
• Moje dijete je nacrtalo.

This is a very common Croatian verb-pair question.

• crtati = to draw, usually imperfective
• nacrtati = to draw / to finish drawing, perfective

In this sentence, nacrtao is used because the action is seen as completed: he drew the shapes.

Very roughly:

• Cijeli dan je crtao. = He was drawing all day.
• Nacrtao je trokut. = He drew a triangle. / He finished drawing a triangle.

So nacrtati fits better when talking about a completed result.

They are direct objects, so they are in the accusative case. But these particular nouns are masculine inanimate, and for masculine inanimate nouns, the accusative singular is the same as the nominative singular.

So:

• trokut → accusative trokut
• kvadrat → accusative kvadrat
• pravokutnik → accusative pravokutnik

That is why they look unchanged.

Compare with an animate masculine noun:

• Vidim psa. = I see a dog. Here pas becomes psa in the accusative.

Because this is another verb phrase in the perfect tense:

• je zaboravio = forgot

The sentence really has two past actions:

1. je nacrtao = drew
2. je zaboravio = forgot

The subject moj sin is understood for both verbs, so it does not need to be repeated:

• Moj sin je nacrtao..., ali je zaboravio...

In English, this is like:

• My son drew..., but forgot...

Because Croatian often omits words that are already clear from context.

So instead of saying:

• Moj sin je nacrtao..., ali moj sin je zaboravio...

it is much more natural to say:

• Moj sin je nacrtao..., ali je zaboravio...

The subject is still understood to be my son.

This kind of omission is very common in Croatian.

It means forgot to draw / forgot to make / forgot to put in the final line.

Grammatically:

• zaboraviti = to forget
• after it, Croatian often uses an infinitive
• povući = to pull, draw, make (depending on context)

So:

• zaboravio povući = forgot to draw

In the context of a line, povući crtu is a natural expression for drawing a line.

Because povući takes a direct object, so crta must be in the accusative singular.

Base forms:

• zadnja = last/final
• crta = line

In the accusative singular feminine:

• zadnja → zadnju
• crta → crtu

So:

• povući zadnju crtu = to draw the final line

This is a very useful pattern to notice:

• feminine nouns ending in -a often become -u in the accusative singular.

Example:

• imam knjigu from knjiga
• vidim sestru from sestra

Both can relate to the idea of a line, but crta is often the more natural word for a line that you draw with a pen or pencil.

• crta = a drawn line, stroke
• linija = line in a broader sense; it can also mean a route, a line in geometry, a phone line, etc.

In this sentence, crta sounds very natural because the child is drawing shapes.

• pravokutnik = rectangle
• kvadrat = square

A square is a special kind of rectangle, but Croatian normally uses different everyday words for them, just as English does.

Other useful shape words:

• trokut = triangle
• krug = circle
• kružnica = circumference/circle line
• četverokut = quadrilateral

Yes, Croatian word order is fairly flexible, but this sentence uses a very normal and natural order.

One important point is that je is a clitic, and clitics usually appear in the second position of their clause.

So:

• Moj sin je nacrtao...
• ali je zaboravio...

This is why je comes so early.

You may sometimes see other word orders for emphasis, but the version in the sentence is standard and neutral.

Because Croatian has no articles.

So nouns like:

• trokut
• kvadrat
• pravokutnik
• crtu

can mean a triangle / the triangle, a line / the line, depending on context.

English requires articles, but Croatian usually leaves that information to context.

Yes, in context that is exactly the idea. Literally it means the last line, but when talking about drawing shapes, that last line is effectively the final side needed to complete one of the figures.

So English might express the idea as:

• the last line
• the final line
• the last side

The Croatian wording stays with crta, which is perfectly natural.