Ova majica je od pamuka.

Because majica is a feminine singular noun, and the demonstrative this has to agree with the noun in gender, number, and case.

Here the forms are:

• ovaj = this, for masculine singular
• ova = this, for feminine singular
• ovo = this, for neuter singular

Since majica is feminine, you say ova majica.

A few comparisons:

• ovaj stol = this table
• ova majica = this T-shirt
• ovo selo = this village

Majica is in the nominative singular.

That is because it is the subject of the sentence:

• Ova majica = the thing being talked about
• je od pamuka = is made of cotton

So the basic structure is:

• subject + verb + complement

In Croatian, subjects are normally in the nominative.

Je is the 3rd person singular present tense of the verb biti = to be.

So:

• ja sam = I am
• ti si = you are
• on/ona/ono je = he/she/it is

Since majica is singular, you need je:

• Ova majica je od pamuka.

You cannot normally leave it out in a standard sentence like this, because Croatian needs the verb to be here just like English does in This shirt is made of cotton.

Because Croatian uses the preposition od to express made of / made from in sentences like this.

So the pattern is:

• od + genitive

That means the noun after od must go into the genitive case.

The noun is:

• pamuk = cotton

Its genitive singular is:

• pamuka

So:

• od pamuka = of cotton / made of cotton

You would not say od pamuk, because od requires the genitive.

The dictionary form is pamuk, which is nominative singular.

After the preposition od, Croatian requires the genitive, so pamuk changes to pamuka.

This is very common in Croatian:

• čaša vode = a glass of water
• stol od drveta = a table made of wood
• haljina od svile = a dress made of silk
• majica od pamuka = a T-shirt made of cotton

So one useful thing to remember is:

• od often means you should expect the genitive

In this sentence, the natural meaning is made of cotton.

Word-for-word, od often corresponds to of or from, but in clothing/material contexts, English usually says:

• made of cotton
• or simply cotton

So:

• Ova majica je od pamuka.
  = This T-shirt is made of cotton.

A very natural English translation could also be:

• This T-shirt is cotton.
• This is a cotton T-shirt.

But the Croatian structure is specifically is of/from cotton, meaning made of cotton.

Yes.

• majica usually means T-shirt, top, or a casual shirt-like garment
• košulja usually means a shirt in the sense of a button-up shirt

So if the meaning shown to the learner is This shirt is made of cotton, the Croatian word majica more specifically suggests a T-shirt rather than a formal shirt.

That is useful because English shirt is broader than Croatian majica.

Yes, Croatian word order is more flexible than English, although Ova majica je od pamuka is the most neutral order.

You may also hear:

• Ova je majica od pamuka.
• Od pamuka je ova majica.

These can sound more marked or give special emphasis.

For a learner, the safest neutral version is:

• Ova majica je od pamuka.

So yes, the order can change, but the original sentence is the best standard model to learn first.

A simple pronunciation guide for English speakers is:

• ova ≈ OH-vah
• majica ≈ MYEE-tsah or MAH-yee-tsah
• je ≈ yeh
• od ≈ od
• pamuka ≈ PAH-moo-kah

A smoother full approximation:

• OH-vah MAH-yee-tsah yeh od PAH-moo-kah

A couple of useful notes:

• j in Croatian is pronounced like English y in yes
• c in Croatian is pronounced like ts
• stress is not usually marked in normal writing, so beginners normally just imitate native audio rather than relying on stress rules alone

Yes. That is another very natural way to express the idea.

• Ova majica je od pamuka. = This T-shirt is made of cotton.
• Ova majica je pamučna. = This T-shirt is cotton / cotton-made / made of cotton.

Here pamučna is an adjective meaning cotton.

Both are correct, but there is a slight stylistic difference:

• od pamuka focuses on the material
• pamučna uses an adjective and may sound a bit more compact

Both are useful and common.

The plural would be:

• Ove majice su od pamuka. = These T-shirts are made of cotton.

Changes:

• ova → ove because the noun is now plural
• majica → majice for nominative plural
• je → su because the verb must agree with a plural subject

So this sentence is also a good model for agreement:

• singular: Ova majica je od pamuka.
• plural: Ove majice su od pamuka.