Djevojčica još ne zna sama zavezati vezice na tenisicama.

Djevojčica is the subject of the sentence, so it is in the nominative singular.

• djevojčica = little girl / girl
• Base noun: djevojka = girl, young woman
• Diminutive: djevojčica = little girl

So this sentence is about the little girl doing or not doing something.

Here još means still.

So:

• još ne zna = still doesn’t know how

It adds the idea that this may change later. She does not know how yet.

A very natural English translation would be:

• The little girl still doesn’t know how to...
• or The little girl doesn’t yet know how to...

Because znati here means to know how to do something, not just to know a fact.

• ne zna zavezati vezice = she doesn’t know how to tie shoelaces
• ne može zavezati vezice = she can’t tie shoelaces (maybe physically unable, not allowed, or failing at the moment)

So ne zna focuses on lack of skill/knowledge, not lack of ability in one specific situation.

Sama means by herself / on her own / alone.

It agrees with djevojčica, which is:

• feminine
• singular
• nominative

That is why the form is sama.

Compare:

• dječak sam = the boy himself / alone
• djevojčica sama = the girl herself / alone
• dijete samo = the child itself / alone

In this sentence, sama means that she does not yet know how to tie the laces without help.

After znati, Croatian often uses an infinitive to express knowing how to do something.

So:

• zna plivati = knows how to swim
• zna kuhati = knows how to cook
• zna zavezati vezice = knows how to tie shoelaces

This is very similar to English know how to + verb.

This is about aspect.

• vezati = imperfective
• zavezati = perfective

In this sentence, zavezati is used because it refers to a completed action: tying the laces so that they end up tied.

Very roughly:

• vezati = to be tying / to tie in a general sense
• zavezati = to tie up, to finish tying

With practical tasks like this, Croatian often prefers the perfective when talking about being able to complete the action successfully.

Vezice means laces, and in this context specifically shoelaces.

The singular is:

• vezica = lace, small string, tie

The plural is:

• vezice = laces

Since shoes normally have two laces, the plural is natural here.

Vezice is the accusative plural, but for many feminine nouns like this, the accusative plural looks the same as the nominative plural.

Singular:

• nominative: vezica
• accusative: vezicu

Plural:

• nominative: vezice
• accusative: vezice

Here they are the object of zavezati, so accusative is required.

Because tenisicama is in the locative plural, used after na here.

• tenisice = sneakers / trainers / tennis shoes
• na tenisicama = on the sneakers

The shoelaces are understood as being on the shoes, so Croatian uses:

• na + locative

That gives:

• singular: na tenisici
• plural: na tenisicama

The ending -ama is a very common feminine plural ending in oblique cases.

Croatian often adds this kind of phrase to make the meaning more specific and natural.

• vezice alone could mean different kinds of laces, ties, cords, etc.
• vezice na tenisicama clearly means the shoelaces on the sneakers

In English, we often just say shoelaces, but Croatian commonly expresses it more literally.

Because the word for this kind of footwear is normally used in the plural, just like English shoes in many contexts.

• tenisica = one sneaker
• tenisice = sneakers / a pair of sneakers

Since the sentence refers to the laces on the shoes she is wearing or using, the plural is the most natural choice.

Yes. Croatian word order is fairly flexible, although some versions sound more natural than others.

The original:

• Djevojčica još ne zna sama zavezati vezice na tenisicama.

This is natural and clear.

Other possible orders include:

• Djevojčica sama još ne zna zavezati vezice na tenisicama.
• Još djevojčica ne zna sama zavezati vezice na tenisicama.
  This is possible in a special context, but less neutral.
• Djevojčica još sama ne zna zavezati vezice na tenisicama.

Changing the order can shift the emphasis:

• još emphasizes still/not yet
• sama emphasizes by herself
• putting something earlier often gives it more focus

For a learner, the original sentence is a very good neutral model.

Yes, that is possible, but it changes the feel slightly.

• zavezati focuses more on successfully getting them tied
• vezati is more general, focusing on the activity itself

In many everyday situations, native speakers may use either, but zavezati is especially natural when talking about a child learning to complete the task properly.

Here it is much closer to by herself / on her own than to alone.

So the idea is:

• not just that no one else is present
• but that she cannot do it without someone’s help

That is an important nuance. In this sentence, sama is really about independence in performing the action.