Na prvom satu nisam bio razumio razliku između rečenica „radio sam“ i „bio sam radio“.

With sat in the sense of a school period or lesson, Croatian normally uses na: na satu, na predavanju, na seminaru.

So na prvom satu means in/at the first class.

Grammatically:

• na with a stationary meaning takes the locative
• prvi sat therefore becomes na prvom satu

U prvom satu would usually mean something more like within the first hour of some process, not during the first lesson.

Here sat means a class period / lesson / school hour, not a clock.

So:

• prvi sat = the first lesson / first class period
• na prvom satu = in the first lesson

This is very common in school-related Croatian.

Because razliku is the accusative singular form of razlika.

The verb razumjeti takes a direct object, and direct objects are often in the accusative:

• razlika = difference, as a dictionary form
• razliku = difference, as the object

So in this sentence, the speaker did not understand the difference, which is why Croatian uses razliku.

The preposition između normally takes the genitive.

So:

• rečenica = sentence, the basic form
• rečenica = also the genitive plural form

That can look confusing, because the nominative singular and genitive plural happen to be spelled the same here.

So između rečenica means between the sentences.

Because Croatian often leaves out subject pronouns when the verb already shows who the subject is.

Here nisam already tells you it is first person singular: I did not / I was not.

So ja is unnecessary unless you want emphasis or contrast:

• Ja nisam bio razumio, ali ona jest. = I hadn’t understood, but she had.

This omission of subject pronouns is very normal in Croatian.

It is the pluperfect.

The pluperfect expresses an action that happened before another point in the past.

Structure:

• negative auxiliary nisam
• past participle of biti: bio
• past participle of the main verb: razumio

So nisam bio razumio means roughly I had not understood.

It is grammatical, but it is less common in everyday speech than the ordinary perfect.

In conversation, many speakers would simply say:

• Na prvom satu nisam razumio razliku...

and let the context show the time relationship.

The pluperfect, such as nisam bio razumio or bio sam radio, is more likely to appear:

• in writing
• in storytelling
• in more careful or stylistically marked speech
• when the speaker wants to make the sequence of past events especially clear

So a learner should recognize it, but should also know that it is not the most frequent everyday form.

The difference is mainly one of tense.

• radio sam = perfect
• bio sam radio = pluperfect

Very roughly:

• radio sam can mean I worked, I was working, or sometimes I have worked, depending on context
• bio sam radio means I had worked or I had been working before something else happened

So bio sam radio places the action further back in the past than radio sam.

That is exactly why the sentence mentions those two forms: the speaker did not understand the difference between the ordinary past form and the earlier-past form.

Because both are masculine singular past participles.

They agree with the speaker’s gender:

• male speaker: razumio, radio, bio
• female speaker: razumjela, radila, bila

So if the speaker were female, the sentence would begin:

• Na prvom satu nisam bila razumjela...

And the quoted examples would be:

• radila sam
• bila sam radila

This kind of gender agreement is a normal feature of Croatian past forms.

Croatian has fairly flexible word order, but auxiliaries and clitics follow strong patterns.

In the pluperfect, the usual learner-friendly pattern is:

• bio sam radio
• bila je došla
• bili smo vidjeli

In the negative, the fused negative auxiliary comes first:

• nisam bio razumio
• nije bila došla

So the order in your sentence is normal and standard.

A good practical rule is:

• affirmative pluperfect: bio/bila/bilo + short form of biti + participle
• negative pluperfect: nisam/nisi/nije... + bio/bila/bilo + participle

Even though Croatian word order can vary for style or emphasis, these are the safest patterns for learners to use.