Kupila sam novi otirač, jer je stari bio mali za naš ulaz.

Kupila sam is the past tense in Croatian.

• kupila = the l-participle form, here feminine singular
• sam = I am, used as an auxiliary to form the past tense

So:

• kupio sam = I bought (masculine speaker)
• kupila sam = I bought (feminine speaker)

Croatian past tense usually needs both parts.

That ending shows the gender and number of the speaker in the past tense.

Because the speaker is feminine singular, we get:

• kupila sam = I bought (female speaker)

Compare:

• kupio sam = I bought (male speaker)
• kupili smo = we bought (mixed group or masculine plural)
• kupile smo = we bought (all-female group)

So the verb agrees with the subject’s gender in the past tense.

In normal Croatian, sam usually comes in the second position in the clause. It is a clitic, which means it likes to stand near the beginning, usually after the first stressed element.

So:

• Kupila sam novi otirač. = natural
• Sam kupila novi otirač. = not normal standard word order

You will often see this second-position behavior with words like:

• sam, si, je, smo, ste, su
• se
• short object pronouns like ga, je, mi, ti

Because novi must agree with otirač in gender, number, and case.

Here:

• otirač = masculine singular
• it is the direct object
• with this verb, it appears in the accusative singular
• for an inanimate masculine noun, the accusative usually looks the same as the nominative

So:

• nominative: novi otirač
• accusative: novi otirač

That is why the form stays novi.

It is in the accusative, because it is the thing being bought.

In the sentence:

• Kupila sam novi otirač = I bought a new doormat

The doormat is the direct object, so Croatian uses the accusative.

Because otirač is a masculine inanimate noun, its accusative singular is the same as its nominative singular:

• nominative: otirač
• accusative: otirač

Because jer means because and introduces a subordinate clause. In Croatian, that clause is normally separated by a comma.

So:

• Kupila sam novi otirač, jer je stari bio mali za naš ulaz.

This is standard punctuation.

Jer means because.

In this sentence:

• jer je stari bio mali za naš ulaz
• because the old one was too small for our entrance

It is a very common everyday conjunction.

Another common word is zato što, which also means because:

• Kupila sam novi otirač zato što je stari bio mali...

Both are common, but jer is shorter and very frequent in speech and writing.

Here je is another auxiliary verb, from biti (to be), used to form the past tense.

The clause:

• stari je bio mali = the old one was small

More literally:

• bio = been / was (masculine singular)
• je = is, used as the auxiliary for he/it

Together, je bio means was.

Because stari refers to otirač, which is masculine singular, we get:

• stari je bio...

Croatian often leaves out a noun when it is already understood from context.

So stari here means:

• the old one
• more specifically, the old doormat

This is very natural in Croatian, just like in English:

• I bought a new one because the old one was too small.

The omitted noun is understood from the earlier part of the sentence.

Because it stands in place of otirač, and otirač is a masculine noun.

So even when the noun is omitted, the adjective still shows its gender:

• stari = old one (masculine)
• stara = old one (feminine)
• staro = old one (neuter)

Since otirač is masculine, stari must also be masculine.

It is in the nominative singular masculine, because it is effectively the subject of the clause:

• stari bio mali
• the old one was small

Even though the noun is omitted, stari functions like a noun here, so it appears in the nominative as the subject.

Mali means small, and it is the ordinary, common adjective to use here.

So:

• bio mali = was small

Croatian has other related words, such as:

• malen = small, a bit more descriptive or stylistic
• premalen = too small

In this sentence, mali is the straightforward basic choice.

It often could be premali, and that would be very natural:

• jer je stari bio premali za naš ulaz

That means more explicitly:

• because the old one was too small for our entrance

But Croatian can also use just mali when the context already makes the comparison clear. If the meaning shown to the learner is too small, premali would be a very common alternative.

It means for our entrance / entryway.

Breakdown:

• za = for
• naš = our
• ulaz = entrance, entryway, doorway area, building entrance, depending on context

So:

• mali za naš ulaz = too small for our entrance

This phrase shows what the old doormat was too small for.

Because the preposition za here takes the accusative case when it means for.

So:

• za naš ulaz = for our entrance

Here:

• ulaz is masculine singular
• accusative singular of an inanimate masculine noun looks like the nominative
• therefore: ulaz
• the adjective/possessive agrees: naš

Compare with dative/locative forms, which would be different:

• našem ulazu = to/about/in our entrance

But that is not what za requires in this sentence.

In Croatian, possessive words like my, your, our behave like adjectives and agree with the noun.

So:

• naš ulaz = our entrance (masculine singular)
• naša kuća = our house (feminine singular)
• naše dijete = our child (neuter singular)

That is why naš changes form depending on the noun it describes.

Croatian word order is fairly flexible, but some versions are more natural than others.

Standard neutral order:

• Kupila sam novi otirač, jer je stari bio mali za naš ulaz.

You might also hear:

• Novi otirač sam kupila, jer je stari bio mali za naš ulaz.
  This puts emphasis on novi otirač.

But the clitics still follow their usual pattern:

• sam
• je

So word order can change for emphasis, but not completely freely.

They are not the same.

• ulaz = entrance, entryway, the area you enter through, sometimes the building entrance
• vrata = door

A doormat can be described in relation to either one, depending on meaning:

• mali za naš ulaz = too small for our entrance area
• mali za naša vrata = too small for our door

So ulaz may suggest a broader entrance space, not just the physical door itself.

Yes, otirač is a normal word for a doormat.

It comes from the idea of wiping or scraping dirt from shoes. In everyday use, it refers to the mat placed by a door or entrance.

So:

• novi otirač = a new doormat

Grammatically, yes, but in natural English you would usually say:

• because the old one was too small for our entrance
• or because the old one was too small for the doorway

English normally wants too small here, because the reason for buying a new one is that the old one was not large enough. Croatian can sometimes leave that idea a little more implicit with mali, depending on context.