U perfektu često griješim, pa moram još vježbati i prezent i perfekt.

Because u here means in, and after u (when it means location or within something) Croatian normally uses the locative case.

• perfekt = the perfect tense
• u perfektu = in the perfect tense

So U perfektu često griješim literally means I often make mistakes in the perfect tense.

You can compare:

• u školi = in school
• u gradu = in the city
• u perfektu = in the perfect tense

The form perfektu is the locative singular of perfekt.

Yes, griješiti means to make mistakes / to be wrong / to err.

So:

• često griješim = I often make mistakes / I am often wrong

Croatian often uses griješiti where English uses a phrase like make mistakes.

You could also hear:

• radim greške = I make mistakes

But griješim is very natural and compact here.

Because Croatian usually does not need the subject pronoun when the verb ending already shows who the subject is.

• griješim = I make mistakes
• moram = I must / I have to

The -m ending tells you the subject is I.

So:

• Ja često griješim is possible, but
• Često griješim is more normal unless you want emphasis.

Croatian drops subject pronouns much more often than English.

Here pa means something like:

• so
• therefore
• and so

It links the two ideas:

• U perfektu često griješim = I often make mistakes in the perfect tense
• pa moram još vježbati... = so I still have to practice...

It is a very common conversational connector in Croatian.

Here još means still.

So:

• moram još vježbati = I still have to practice

Depending on context, još can also mean:

• more
• another
• yet
• still

But in this sentence, still is the best fit.

Because after morati (must / have to), Croatian uses the infinitive.

• moram vježbati = I have to practice
• moram učiti = I have to study
• moram ići = I have to go

So vježbati is the infinitive to practice.

This structure means both X and Y.

• i prezent i perfekt = both the present tense and the perfect tense

Croatian often repeats i in this pattern:

• i mama i tata = both mum and dad
• i danas i sutra = both today and tomorrow
• i prezent i perfekt = both the present and the perfect

So the repeated i is completely normal.

They are functioning as direct objects of vježbati, so they are in the accusative case.

For these nouns, the accusative singular looks the same as the nominative singular because they are inanimate masculine nouns.

So:

• nominative: prezent, perfekt
• accusative: prezent, perfekt

That is why you see:

• vježbati prezent
• vježbati perfekt

But after u, you saw perfektu, because that required the locative.

They are the names of verb tenses:

• prezent = the present tense
• perfekt = the perfect tense / past tense form commonly used in Croatian

In Croatian grammar terms:

• prezent is used for present-time actions
• perfekt is one of the main past tenses and is very commonly used

For example:

• radim = I work / I am working (prezent)
• radio sam / radila sam = I worked / I was working / I have worked (perfekt)

So the speaker is saying they need more practice with both of those tense forms.

Not completely. Croatian word order is fairly flexible, but the original sentence sounds natural:

• U perfektu često griješim, pa moram još vježbati i prezent i perfekt.

You could rearrange parts for emphasis, for example:

• Često griješim u perfektu...
• Moram još vježbati i prezent i perfekt...

But the original order is smooth and idiomatic.

A useful point: placing često before griješim is very natural, and u perfektu at the front gives the topic first: as for the perfect tense...

Because the noun is in different cases in the two parts of the sentence.

1. u perfektu

   • after u
   • needs the locative
   • means in the perfect tense

2. vježbati perfekt

   • object of vježbati
   • uses the accusative
   • for an inanimate masculine noun, accusative = nominative
   • so it stays perfekt

So the change is grammatical, not a change in meaning.

Yes, there are several natural ways to express the same general idea. For example:

• U perfektu često griješim.
• Često griješim u perfektu.
• Imam problema s perfektom. = I have problems with the perfect tense.
• Moram još vježbati perfekt.
• Moram još vježbati i prezent i perfekt.

The given sentence is nice because it clearly links the problem and the solution:

• I make mistakes
• so I need more practice

A very natural English translation would be:

• I often make mistakes in the perfect tense, so I still need to practise both the present tense and the perfect.

Depending on style, you could also say:

• I often make mistakes with the perfect tense, so I still need to practise both the present and the perfect.
• I often get the perfect tense wrong, so I still need to practise both the present and the perfect.

All of these capture the Croatian sentence well.