Moja prijava za tečaj još nije potpuna.

Because prijava is a feminine singular noun, and the possessive adjective moj has to agree with it.

So:

• moj = masculine singular
• moja = feminine singular
• moje = neuter singular

Since prijava is feminine, you say moja prijava = my application.

In this sentence, prijava means application or registration/application form, depending on context.

It can refer to things like:

• an application for a course
• a sign-up/registration
• a submitted form

In other contexts, prijava can also mean things like a report or complaint, so the exact meaning depends on what follows it. Here, because of za tečaj, it clearly means an application for a course.

It is in the nominative singular.

Why?

Because prijava is the subject of the sentence:

• Moja prijava = the thing being talked about
• nije potpuna = is not complete

So the structure is basically:

• My application
  • is not complete yet

That makes prijava nominative.

Here za means for, and in this meaning it normally takes the accusative case.

So:

• tečaj = course
• za tečaj = for a course / for the course

This is a very common pattern in Croatian:

• prijava za posao = application for a job
• karta za koncert = ticket for the concert
• novac za stan = money for the apartment

So prijava za tečaj is a normal noun phrase meaning application for a course.

Because tečaj is a masculine inanimate noun, and in the singular, the accusative is often the same as the nominative.

So:

• nominative: tečaj
• accusative: tečaj

That is why you do not see a change in form, even though the case is accusative.

This is very common with masculine inanimate nouns in Croatian.

Here još means still or yet, depending on how you translate it into English.

So:

• još nije potpuna = is still not complete
• or is not complete yet

It adds the idea that the situation is continuing up to now.

Compare:

• Moja prijava nije potpuna. = My application is not complete.
• Moja prijava još nije potpuna. = My application is still not complete / My application is not complete yet.

Because nije is the standard negative form of je (is) from the verb biti (to be).

So:

• je = is
• nije = is not

In Croatian, this is normally written as one word.

Examples:

• On je ovdje. = He is here.
• On nije ovdje. = He is not here.

So nije potpuna literally means is not complete.

Because potpuna is an adjective, and adjectives in Croatian must agree with the noun they describe in:

• gender
• number
• case

Since prijava is feminine singular nominative, the adjective must also be feminine singular nominative:

• masculine: potpun
• feminine: potpuna
• neuter: potpuno

So:

• prijava je potpuna = the application is complete
• tečaj je potpun would be masculine
• pismo je potpuno would be neuter

Yes, you could say Moja prijava je nepotpuna, and it would mean roughly My application is incomplete.

But there is a slight difference in feel:

• nije potpuna = is not complete
• je nepotpuna = is incomplete

Both are correct.
The version in your sentence, još nije potpuna, sounds very natural when emphasizing that something has not been completed yet.

Croatian word order is fairly flexible, but some orders sound more natural than others.

The original sentence:

• Moja prijava za tečaj još nije potpuna.

is very natural and neutral.

You could also hear:

• Moja prijava za tečaj nije još potpuna.

but this is less natural in standard usage.

You might also move words for emphasis, for example:

• Još moja prijava za tečaj nije potpuna.

This sounds marked and would only be used in special contexts.

So for a learner, the safest and most natural version is the original one.

Because Croatian does not have articles like English a/an/the.

So Croatian often expresses definiteness through context, word order, possession, or demonstratives if needed.

Here:

• moja prijava already clearly means my application

That makes an article unnecessary. English needs my application, but Croatian simply uses the possessive adjective and no article.

Yes, tečaj is a very common word for a course, especially a practical or organized course such as:

• a language course
• a computer course
• a training course

For example:

• tečaj hrvatskog jezika = Croatian language course
• tečaj vožnje = driving course

So prijava za tečaj is a very natural phrase.

A rough pronunciation guide is:

MO-ya pree-YA-va za TE-chai yosh NEE-ye pot-POO-na

A few useful sound notes:

• j is pronounced like English y
• č sounds like ch in church
• š sounds like sh in shoe
• nije sounds roughly like NEE-ye

So:

• moja → MO-ya
• prijava → pree-YA-va
• tečaj → TE-chai
• još → yosh
• nije → NEE-ye
• potpuna → pot-POO-na

The structure is:

• Moja prijava za tečaj = subject noun phrase
• još = adverb
• nije = verb
• potpuna = predicate adjective

So literally:

• My application for the course still is-not complete

A more natural English rendering is:

• My application for the course is still not complete
• or My application for the course isn’t complete yet

This is a very common Croatian pattern:

[subject] + [adverb] + [nije] + [adjective]

For example:

• Dokument još nije gotov. = The document is still not finished.
• Soba još nije čista. = The room is still not clean.