Kad se prijavljuješ na tečaj, moraš pažljivo pročitati sve uvjete.

Kad means when.

In this sentence, Kad se prijavljuješ na tečaj... means When you sign up for a course...

Kad and kada usually mean the same thing.

• kad is shorter and very common in everyday speech
• kada can sound a bit more formal or emphatic

So both are correct:

• Kad se prijavljuješ na tečaj...
• Kada se prijavljuješ na tečaj...

Because the verb is prijaviti se / prijavljivati se / prijavljivati in the sense of to sign up / to register.

In Croatian, many verbs are used with se, and you often just have to learn them as part of the verb. Here:

• prijaviti se = to sign up, to register oneself
• prijavljuješ se = you are signing up / you sign up

So se is not optional here.
Without it, prijaviti usually means something more like to report or to submit/register something depending on context.

Examples:

• Prijavio sam se na tečaj. = I signed up for the course.
• Prijavio sam problem. = I reported the problem.

This is mainly about aspect.

Croatian verbs often come in pairs:

• prijavljivati se = imperfective
• prijaviti se = perfective

Here, prijavljuješ is the present tense of the imperfective verb prijavljivati se.

Why use it here? Because the sentence expresses a general rule or habitual situation:

• When you sign up for a course, you must...

In this kind of whenever/when statement, Croatian often uses the imperfective present:

• Kad se prijavljuješ... = When(ever) you are signing up / when you sign up

If you said Kad se prijaviš..., that would sound more like:

• When you sign up in a more specific, completed-event sense

Both can be possible in some contexts, but prijavljuješ fits the idea of a general instruction very naturally.

Tečaj is in the accusative singular here.

The preposition na often takes:

• accusative when there is movement onto / to / toward something
• locative when it means being on / at something

With prijaviti se na something, Croatian uses na + accusative:

• prijaviti se na tečaj = to sign up for a course
• prijaviti se na ispit = to register for an exam

For the noun tečaj, nominative and accusative singular look the same:

• nominative: tečaj
• accusative: tečaj

So the form does not change, but the case function is accusative.

Moraš is from morati, which means must / have to.

Croatian commonly uses:

• morati + infinitive

So:

• moraš pročitati = you must read
• more literally: you must to-read

Examples:

• Moraš doći. = You must come.
• Moram učiti. = I have to study.

In the sentence:

• moraš pažljivo pročitati sve uvjete = you must carefully read all the terms/conditions

This is another aspect question.

• čitati = imperfective, to read in an ongoing or general sense
• pročitati = perfective, to read through / to finish reading

Since the sentence means you must read all the conditions completely, Croatian uses the perfective verb:

• pročitati sve uvjete = read all the conditions fully

Compare:

• Moram čitati. = I have to read.
  (general activity)
• Moram pročitati ovaj članak. = I have to read this article.
  (finish it)

So pročitati emphasizes completion.

Pažljivo means carefully.

It is an adverb, so it describes how you must read:

• moraš pažljivo pročitati = you must read carefully

Its position is fairly flexible, though some placements sound more natural than others:

• Moraš pažljivo pročitati sve uvjete.
• Moraš pročitati sve uvjete pažljivo.
  (possible, but less natural here)
• Sve uvjete moraš pažljivo pročitati.
  (more emphasis on all the conditions)

The version in your sentence is very natural.

Because sve uvjete is the direct object of pročitati, so it must be in the accusative plural.

The basic dictionary form is:

• uvjet = condition, term

Plural:

• nominative plural: uvjeti = conditions
• accusative plural: uvjete

So:

• svi uvjeti = all the conditions
  (subject form, nominative)
• pročitati sve uvjete = to read all the conditions
  (object form, accusative)

The word sve here agrees with the noun in case/number/gender:

• sve uvjete = all the conditions

Uvjet means condition, and in this context uvjeti usually means:

• terms
• conditions
• sometimes requirements, depending on context

So sve uvjete could be translated as:

• all the conditions
• all the terms
• all the requirements

If this is about signing up for a course, English would often naturally say:

• read all the terms and conditions or
• read all the requirements

It most naturally sounds like a general situation:

• Whenever you sign up for a course, you must carefully read all the conditions.

That is because of:

• kad = when / whenever
• prijavljuješ = imperfective present, often used for repeated or general actions

If the speaker wanted one specific future event, they might phrase it differently depending on style and context, for example:

• Kad se prijaviš na tečaj, moraš...
• Kad budeš prijavljivao/prijavljivala se...
  in some contexts, though that is less simple and not the default here

So your sentence reads like a general instruction or rule.

No, Croatian word order is fairly flexible, but different orders change the focus or emphasis.

The original sentence:

• Kad se prijavljuješ na tečaj, moraš pažljivo pročitati sve uvjete.

Other possible orders:

• Kad se prijavljuješ na tečaj, sve uvjete moraš pažljivo pročitati.
  Emphasizes all the conditions
• Moraš pažljivo pročitati sve uvjete kad se prijavljuješ na tečaj.
  Same meaning, but the time clause is moved to the end

The original order is neutral and natural.

A rough pronunciation guide:

• tečaj ≈ TEH-chai

  • č sounds like ch in chocolate
  • aj sounds like eye in many Croatian words

• uvjete ≈ OO-vyeh-teh

  • u = oo
  • vj makes a vy kind of sound together
  • final e is clearly pronounced, not silent

Also:

• prijavljuješ is roughly pree-YAH-vlyoo-yesh
• pažljivo is roughly PAHZH-lyee-vo

Exact pronunciation varies a little by region, but these approximations are helpful for an English speaker.

Yes, but the meaning changes slightly.

• moraš = you must / you have to
• trebaš = you should / you need to

Compare:

• Moraš pažljivo pročitati sve uvjete.
  = You must carefully read all the conditions.
  Stronger, more obligatory.
• Trebaš pažljivo pročitati sve uvjete.
  = You should / need to carefully read all the conditions.
  Slightly softer.

So moraš is better if the speaker wants to express a firm rule or requirement.