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

Because the speaker is female.

In Croatian past tense, the main verb agrees with the gender and number of the subject:

• kupio sam = I bought (male speaker)
• kupila sam = I bought (female speaker)

So Kupila sam novi otirač means I bought a new doormat, said by a woman.

If a man were speaking, it would be:

• Kupio sam novi otirač.

Croatian often leaves out subject pronouns when they are clear from the verb form.

So:

• Kupila sam literally contains the information bought + am, which already tells you I.
• Adding ja is possible, but usually only for emphasis or contrast.

Compare:

• Kupila sam novi otirač. = I bought a new doormat.
• Ja sam kupila novi otirač. = I bought the doormat. / It was I who bought it.

This is very normal in Croatian.

Croatian past tense is often built with:

• the past participle of the main verb
• plus a form of biti (to be)

Here:

• kupila = past participle of kupiti (to buy), feminine singular
• sam = I am, used here as an auxiliary

Together:

• kupila sam = I bought

Other examples:

• kupio sam = I bought (male speaker)
• kupila je = she bought
• kupili smo = we bought (mixed group or all male)
• kupile smo = we bought (all female)

Normally, no.

In standard Croatian, the auxiliary sam is a clitic, and clitics usually cannot stand in the first position of the sentence.

That is why you say:

• Kupila sam novi otirač.
• Ja sam kupila novi otirač.

but not normally:

• Sam kupila novi otirač.

Croatian has fairly strict rules about where short unstressed words like sam, si, je, ga, se go.

It is in the accusative singular, because it is the direct object of kupila sam.

The noun is:

• otirač = doormat

Since otirač is a masculine noun referring to a non-living thing, its accusative singular is the same as its nominative singular:

• otirač (nominative)
• otirač (accusative)

The adjective must match the noun in gender, number, and case:

• novi otirač = new doormat

So:

• Kupila sam novi otirač = I bought a new doormat

It is originally an adjective, but here it is being used like a noun.

Literally, stari means old, but in this sentence it means:

• the old one

So:

• novi otirač = the new doormat
• stari = the old one / the old doormat

This is common in Croatian. The noun can be omitted if it is obvious from context.

A fuller version would be:

• jer je stari otirač bio premalen za naš ulaz

But Croatian often leaves out the repeated noun.

Je is the 3rd person singular form of biti (to be) used as an auxiliary in the past tense.

The phrase bio premalen means was too small, and it is made of:

• bio = past participle of biti
• je = auxiliary is/has, here helping form the past tense

So:

• stari je bio premalen or, with clitic placement after jer:
• jer je stari bio premalen

Both are about the old doormat being too small.

Since stari refers to a masculine noun (otirač), the participle is masculine:

• bio = masculine
• bila = feminine
• bilo = neuter

Because je is a clitic, and clitics usually go in the second position in their clause.

After jer (because), the clitic often comes immediately after it:

• jer je stari bio premalen

This is a very typical Croatian pattern.

You may also hear or see slightly different word orders in real usage, but for a learner, jer je stari bio premalen is a very good standard model.

Malen means small.

The prefix pre- often gives the meaning too in Croatian:

• malen = small
• premalen = too small

So:

• bio premalen = was too small

This prefix is very useful. For example:

• skup = expensive

• preskup = too expensive

• dug = long
• predug = too long

Because the phrase is za naš ulaz, and the preposition za here takes the accusative case.

So:

• ulaz = entrance
• naš ulaz = our entrance (accusative singular here, same form as nominative because it is masculine inanimate)

The whole phrase means:

• za naš ulaz = for our entrance

If you used a different preposition, the case might change. For example:

• našem ulazu would be dative/locative singular, not what is needed here.

Because it must agree with ulaz.

The noun ulaz is:

• masculine
• singular

So the possessive adjective must also be masculine singular:

• naš ulaz = our entrance

Compare:

• naša kuća = our house (feminine)
• naše dijete = our child (neuter)

In this sentence, ulaz is masculine, so naš is the correct form.

Because jer introduces a subordinate clause meaning because, and in standard Croatian it is normally separated by a comma.

So:

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

This punctuation is standard and natural.

Here are the basic dictionary forms:

• kupila → kupiti = to buy
• otirač = doormat
• stari → star = old
• bio → biti = to be
• premalen → premalen / based on malen = too small / small
• naš = our
• ulaz = entrance

This is helpful because Croatian words change form a lot, so learners should get used to recognizing the dictionary form behind the sentence form.

Only the parts that agree with the speaker or noun would change where necessary.

You would say:

• Kupio sam novi otirač, jer je stari bio premalen za naš ulaz.

The only change is:

• kupila sam → kupio sam

That is because the speaker is now male.

The rest stays the same because:

• otirač is still masculine
• stari still refers to otirač
• bio premalen still agrees with the old doormat, not with the speaker

Yes, that is a very common pattern in Croatian.

Most nouns ending in a consonant are masculine:

• otirač = doormat
• ulaz = entrance
• grad = city
• stol = table

That is why you get masculine agreement:

• novi otirač
• stari
• bio premalen

Of course, there are exceptions in the language, but as a general rule this pattern is very useful.