Kad je buka u učionici, teže razumijem učiteljicu.

Yes. Kad means when in this sentence.

• Kad je buka u učionici = When there is noise in the classroom
• Croatian also has kada, which means the same thing here.
• Kad is very common in everyday speech and writing, while kada can sound a bit more formal or more explicit.

So both of these work:

• Kad je buka u učionici, teže razumijem učiteljicu.
• Kada je buka u učionici, teže razumijem učiteljicu.

Yes, literally it is something like when noise is in the classroom, but the natural English translation is when there is noise in the classroom.

Here:

• je = is
• buka = noise

Croatian often uses biti (to be) in places where English uses there is / there are.

So:

• Kad je buka... = When there is noise...

This is normal Croatian, even though it does not match English word-for-word.

Buka is in the nominative singular.

That is because it is the grammatical subject of je:

• buka je = the noise is

The basic dictionary form is:

• buka = noise

So nothing special is happening to the noun here.

Because u can take different cases depending on meaning.

Here it means in the classroom (location, no movement), so Croatian uses the locative:

• u učionici = in the classroom

If you were talking about movement into the classroom, you would use the accusative:

• u učionicu = into the classroom

So the contrast is:

• Ja sam u učionici. = I am in the classroom.
• Idem u učionicu. = I am going into the classroom.

Učionici is the locative singular of učionica (classroom).

The noun is feminine:

• nominative: učionica
• locative: u učionici

Because it comes after u meaning in, the locative is required here.

Teže is the comparative adverb of teško.

• teško = with difficulty / hard
• teže = more difficultly / harder

So:

• teže razumijem = I understand with more difficulty / I understand less easily / I have a harder time understanding

In natural English, we usually say I understand the teacher less easily or I have a harder time understanding the teacher.

You could say teško razumijem učiteljicu, but it means something slightly different.

• teško razumijem = I understand with difficulty
• teže razumijem = I understand more difficultly / less easily / harder

The comparative teže suggests that the noise makes understanding more difficult than usual.

So in this sentence, teže fits very well because it implies a comparison:

• under noisy conditions, understanding becomes harder.

Razumijem is the standard form based on the infinitive razumjeti (to understand).

In standard Croatian:

• razumjeti = to understand
• razumijem = I understand

You may sometimes see shorter-looking forms in other regional varieties or closely related standards, but for Croatian learners, razumijem is the form to learn.

Because Croatian usually does not need subject pronouns when the verb already shows the person clearly.

• razumijem already means I understand

So ja is unnecessary unless you want emphasis or contrast.

Compare:

• Razumijem učiteljicu. = I understand the teacher.
• Ja razumijem učiteljicu. = I understand the teacher (with extra emphasis, maybe contrasting with someone else)

This omission of subject pronouns is very common in Croatian.

Because učiteljicu is the accusative singular, and it is the direct object of razumijem.

• učiteljica = nominative (teacher, subject form)
• učiteljicu = accusative (teacher, object form)

Since the speaker is understanding the teacher, the teacher is the object:

• Razumijem koga? — učiteljicu
• I understand whom? — the teacher

So Croatian changes the ending to show that role.

Yes. Učiteljica means a female teacher.

So:

• učiteljica = female teacher
• učiteljicu = female teacher in the accusative

If the teacher were male, you would say:

• Kad je buka u učionici, teže razumijem učitelja.

Here:

• učitelj = male teacher
• učitelja = accusative singular

Yes. It is a subordinate clause.

It contains:

• kad = subordinating conjunction (when)
• je = verb
• buka = subject
• u učionici = location

Then the main clause is:

• teže razumijem učiteljicu

So the full sentence has this structure:

• [Kad je buka u učionici], [teže razumijem učiteljicu].

Because Croatian normally uses a comma to separate a subordinate clause from the main clause, especially when the subordinate clause comes first.

So:

• Kad je buka u učionici, teže razumijem učiteljicu.

That comma is standard and expected.

Yes. Croatian word order is fairly flexible.

You could also say:

• Teže razumijem učiteljicu kad je buka u učionici.

That is also natural.

The original version puts the when-clause first, which gives the condition or situation before the main point. The second version starts with the main point first.

Both are correct; the difference is mostly one of emphasis and flow.

Not exactly.

• kad = when
• ako = if

So:

• Kad je buka u učionici... = When there is noise in the classroom...
• Ako je buka u učionici... = If there is noise in the classroom...

Kad suggests this happens as a real situation or repeated circumstance.
Ako presents it more as a condition.

In your sentence, kad is the more natural choice.

Most naturally, it sounds habitual or generally true:

• When there is noise in the classroom, I understand the teacher less easily.

Croatian present tense often does this very naturally. It can describe:

• a general truth,
• a repeated situation,
• or a usual experience.

So the sentence is not necessarily about one single moment. It sounds like a general pattern.