Ove patike su lepe, ali ne znam da li je ova veličina dobra.

Because Serbian demonstratives like this/these must agree with the noun in gender, number, and case.

• patike is feminine plural, so you use ove = these
• veličina is feminine singular, so you use ova = this

So:

• ove patike = these sneakers
• ova veličina = this size

Patike means sneakers / trainers and is plural here because we normally talk about shoes as a pair.

The singular is patika, meaning one sneaker/shoe.

So:

• patika = one sneaker
• patike = sneakers / a pair of sneakers

That is very natural in Serbian.

Because adjectives in Serbian must agree with the noun they describe.

Here, patike is:

• feminine
• plural
• nominative

So the adjective lep changes to lepe.

A few forms of lep are:

• lep = masculine singular
• lepa = feminine singular
• lepo = neuter singular
• lepe = feminine plural

So Ove patike su lepe means These sneakers are beautiful/nice.

Because su is the 3rd person plural form of biti (to be) in the present tense.

The subject is ove patike = these sneakers, which is plural, so the verb must also be plural:

• je = is
• su = are

So:

• Ova patika je lepa. = This sneaker is nice.
• Ove patike su lepe. = These sneakers are nice.

Da li introduces a yes/no question, including an indirect question like whether/if in English.

In this sentence:

• ne znam da li... = I don't know if / whether...

So:

• Ne znam da li je ova veličina dobra. = I don’t know if this size is good/right.

This is one of the most common Serbian ways to say if/whether in this kind of sentence.

After da li, Serbian normally places the finite verb immediately after it.

So the natural order is:

• da li je ova veličina dobra

not usually:

• da li ova veličina je dobra

The second version sounds unnatural to most speakers.

A good pattern to remember is:

• da li + verb + subject/complement

For example:

• Da li je ovo tačno? = Is this correct?
• Ne znam da li je ovo tačno. = I don’t know if this is correct.

Because the two parts talk about different things:

1. Ove patike su lepe
   The subject is patike (sneakers) → plural → su, lepe

2. ...da li je ova veličina dobra
   The subject is veličina (size) → singular → je, dobra

So the grammar changes because the noun being talked about changes.

Yes, dobra literally means good, but in this shopping context it usually means:

• the right size
• suitable
• a good fit

So da li je ova veličina dobra is not really about moral goodness. It means something more like:

• whether this size is right
• whether this size works
• whether this is a good size

Yes. In shopping, especially for shoes, Serbian speakers often use broj as well.

• veličina = size
• broj = number/size

So you may hear:

• Ne znam da li je ovaj broj dobar.

That also means I don’t know if this size is right.

Still, ova veličina is completely correct and natural.

Because Serbian has no articles like English a/an/the.

English needs articles:

• These sneakers are nice, but I don’t know if this size is good.

Serbian does not. Instead, meaning comes from context or from words like demonstratives:

• ove = these
• ova = this

So Serbian can be specific without using articles.

Ali means but.

It connects two contrasting ideas:

• Ove patike su lepe = These sneakers are nice
• ali ne znam... = but I don’t know...

So ali works very much like English but and often appears in the same kind of position between two clauses.

Grammatically, lepe comes from lep, which often means:

• beautiful
• pretty
• nice

In a sentence about shoes, English would usually sound more natural with nice or pretty rather than beautiful.

So:

• Ove patike su lepe
  most naturally = These sneakers are nice / pretty

The exact English wording depends on tone and context.

Both main noun phrases are in the nominative, because they are the subjects of their clauses.

• Ove patike su lepe.

  • patike = nominative plural

• ...da li je ova veličina dobra.

  • veličina = nominative singular

That is why the demonstratives and adjectives also appear in nominative forms:

• ove
• lepe
• ova
• dobra

So this sentence is a good example of basic subject + to be + adjective structure in Serbian.