Učiteljica kaže da je ispravak dobar, ali da sam u zadnjem retku opet napravila tipfeler.

Because both parts after kaže are subordinate clauses:

• da je ispravak dobar
• ali da sam u zadnjem retku opet napravila tipfeler

Croatian often repeats da when joining two reported statements like this. In English, we might say:

• The teacher says that the correction is good, but that I made a typo again...

The second da is not strange or redundant; it neatly introduces the second clause.

These are both present-tense forms of biti used as auxiliaries:

• je = is
• sam = am

They match different subjects:

• ispravak is the subject of je dobar → the correction is good
• the speaker is the subject of sam napravila → I made

So:

• da je ispravak dobar = that the correction is good
• da sam ... napravila = that I ... made

Because the speaker is female.

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

• napravila = a woman saying I made
• napravio = a man saying I made

So this sentence tells you that the person speaking about themselves is female.

Because dobar agrees with ispravak, which is masculine singular.

Croatian adjectives agree with the noun they describe in gender, number, and case:

• masculine singular: dobar
• feminine singular: dobra
• neuter singular: dobro

Since ispravak is masculine, you get ispravak dobar.

Ispravak can mean correction, corrected version, or sometimes make-up exam/test, depending on context.

In this sentence, it most likely means something like:

• a corrected piece of writing
• a correction someone made
• a revised version

Since the teacher says it is good, and then mentions a typo in the last line, the idea is probably your correction/revised text is good, but you still made a typo.

It is in the locative because u here means in in the sense of location.

When u means in/at a place, it usually takes the locative:

• u zadnjem retku = in the last line

Both words are in the locative singular:

• zadnji → zadnjem
• redak → retku

So the phrase literally means in the last line.

Yes, this is one of those forms learners often notice.

The dictionary form is redak = line. But in oblique cases, the stem changes:

• nominative: redak
• locative: retku
• genitive: retka

This kind of stem change is normal in Croatian and just has to be learned with the noun. So u zadnjem retku is the correct form, even though it may not look very transparent at first.

Yes, very normal in everyday speech.

Tipfeler means typo. It is a borrowed word and is widely used in informal and semi-formal language. A more formal expression might be something like tipografska pogreška, but in ordinary conversation and classroom comments, tipfeler is completely natural.

So this sentence sounds very natural with tipfeler.

Because je is a clitic, and Croatian clitics tend to go in second position in their clause.

So after da, the clitic comes early:

• da je ispravak dobar
• not normally da ispravak je dobar

The same thing happens in the second clause:

• da sam u zadnjem retku opet napravila tipfeler

Here sam also appears near the beginning because it is a clitic.

This is one of the most important word-order patterns in Croatian.

Because Croatian often leaves out repeated material when it is already understood.

The full structure would be something like:

• Učiteljica kaže da je ispravak dobar, ali kaže i da sam u zadnjem retku opet napravila tipfeler.

But repeating kaže would be less elegant, so Croatian normally shortens it to:

• ... ali da sam ...

This is very common and natural.

Opet means again.

Here it tells you that this was not the first typo:

• opet napravila tipfeler = made a typo again

Its position is fairly natural here, before the main verb phrase. Croatian word order is flexible, so opet can sometimes move for emphasis, but this placement is very standard and neutral.

Because Croatian has no articles like a, an, or the.

Whether something is definite or indefinite is understood from context, word order, or other clues. So:

• učiteljica can mean the teacher or a teacher
• ispravak can mean the correction or a correction
• tipfeler can mean a typo or the typo

In this sentence, English naturally uses the teacher, the correction, and a typo, but Croatian does not need separate words for that.