Pokušavam zapamtiti svaki datum u kalendaru.

Croatian has verb aspect pairs:

• pokušavati – imperfective (ongoing, repeated action)
• pokušati – perfective (one, completed attempt)

Pokušavam comes from pokušavati (imperfective, present tense), and it matches the English “I am trying” – something in progress or happening regularly.

Pokušam (from pokušati) is perfective and is usually not used in the simple present in everyday speech; you more often see it in:

• future: Pokušat ću zapamtiti… – I will try to remember…
• past: Pokušao sam zapamtiti… – I tried to remember…

In Croatian, when you use a verb like pokušavati / pokušati (to try), it is normally followed by another verb in the infinitive, just like English “try to remember”:

• pokušavam zapamtiti – I am trying to remember
• pokušat ću zapamtiti – I will try to remember

So zapamtiti stays in the infinitive because it depends on pokušavam: try + (to) remember.

Both are related to memory, but aspect and nuance are different:

• pamtiti – imperfective

  • to remember (in general), to have something in your memory
  • focuses on the state or ongoing remembering
  • e.g. Još uvijek pamtim taj dan. – I still remember that day.

• zapamtiti – perfective

  • to memorize, to successfully commit something to memory at some point
  • focuses on the moment of learning or achieving the memory
  • e.g. Pokušavam zapamtiti svaki datum. – I am trying to memorize every date.

In your sentence, zapamtiti fits better because you are talking about the process/goal of learning and committing to memory, not just already having the dates remembered.

You can say Pokušavam pamtiti svaki datum, but the meaning shifts slightly:

• pokušavam zapamtiti svaki datum
  – I’m trying to memorize each date (focus on successfully learning them, one by one).

• pokušavam pamtiti svaki datum
  – I’m trying to keep remembering each date (focus on the ongoing state of remembering, not on the act of learning them for the first time).

In most learning/memorizing contexts, zapamtiti is more natural.

Yes, you can also say:

• Pokušavam zapamtiti sve datume u kalendaru.

The difference:

• svaki datum – every date, highlights each item individually
• sve datume – all the dates, highlights the whole set as a group

Both are grammatically correct and close in meaning.
Subtle nuance:

• svaki datum sounds a bit more like you’re going through dates one by one.
• sve datume is a bit more collective: the entire list/calendar as a whole.

Svaki datum is in the accusative singular:

• svaki – nominative/accusative masculine singular form of “every”
• datum → datum (same form in nominative and accusative for inanimate masculine nouns)

It’s accusative because it is the direct object of zapamtiti:

• (Ja) pokušavam zapamtiti koga/što? – svaki datum.
  (I) am trying to remember *what? – every date.*

Even though zapamtiti is in the infinitive, the object behaves as if the verb were fully finite:

• Zapamtit ću svaki datum. – same case (accusative).

In Croatian, u can take either:

• accusative – when there is movement into something (into), or
• locative – when it’s location inside something (in / inside).

Here we have a static location: the dates are in the calendar, not moving into it. So we use locative:

• u kalendaru – in the calendar (locative singular)
• u kalendar – into the calendar (accusative singular), would imply movement into the calendar (e.g. writing something into it).

So u kalendaru is correct in this context.

Croatian is a pro‑drop language: subject pronouns (ja, ti, on, ona, etc.) are often omitted because the verb ending already shows who the subject is.

• pokušavam → the ending -am clearly shows 1st person singular (I).

You could say:

• Ja pokušavam zapamtiti svaki datum u kalendaru.

but the ja is usually only added for emphasis or contrast:

• Ja pokušavam zapamtiti, a ti ne. – I am trying to remember, and you aren’t.

Present tense of pokušavati (imperfective):

• (ja) pokušavam – I try / I am trying
• (ti) pokušavaš – you try / you are trying (singular, informal)
• (on/ona/ono) pokušava – he/she/it tries
• (mi) pokušavamo – we try
• (vi) pokušavate – you try (plural or formal)
• (oni/one/ona) pokušavaju – they try

In your sentence, pokušavam is 1st person singular.

You could form a similar sentence with sjetiti se, but the meaning changes:

• zapamtiti – to memorize / to commit to memory
• sjetiti se – to remember, to recall (something you have already learned)

Example:

• Pokušavam zapamtiti svaki datum u kalendaru.
  – I’m trying to memorize every date (learning them).

• Pokušavam se sjetiti svakog datuma u kalendaru.
  – I’m trying to remember/recall every date (they were already learned, but you’re trying to recall them now).

Also note:

• sjetiti se is reflexive, so you must add se and use the genitive:
  • svakog datuma (not svaki datum) with sjetiti se.

Yes, Croatian word order is relatively flexible, as long as you keep small clitics (like se, ga, ju) in the right place. Some variants:

• Pokušavam zapamtiti svaki datum u kalendaru. – neutral, most natural.
• Pokušavam svaki datum u kalendaru zapamtiti. – still correct; slightly emphasizes svaki datum u kalendaru.
• Svaki datum u kalendaru pokušavam zapamtiti. – emphasizes every date in the calendar.
• Zapamtiti svaki datum u kalendaru pokušavam. – possible, but sounds more poetic or marked.

For everyday speech, the original order is the most typical.