Learn Hardcore Croatian: Upisao sam u kalendar izlet na jezero na sjeveru, ali oluja je promijenila plan. - I wrote a trip to the lake in the north into the calendar, but the storm changed the plan.

In Croatian, the auxiliary verb biti (to be) in the present tense (sam, si, je, smo, ste, su) is a clitic – a short unstressed word that likes to stand in the second position in the clause.

Upisao sam u kalendar izlet…
Here, upisao is the past participle, and sam is the auxiliary: I have / I did (enter).
This is the most neutral order: [Upisao] [sam] [u kalendar…].
Sam upisao u kalendar izlet…
This is unusual or wrong in normal speech/writing, because sam is trying to go first, and clitics almost never stand in the very first position. They want to be second in the sentence (or in the main clause).

So:

Correct, neutral: Upisao sam u kalendar izlet…
Also possible (with some emphasis): U kalendar sam upisao izlet…
(Here u kalendar is first, so sam becomes second.)

Why is it upisao and not something like upisao sam se? What does upisati mean here?

The verb upisati has several uses depending on whether it has an object and/or se:

upisati nešto (u nešto) – to write/enter something (into something)
- Upisao sam u kalendar izlet.
  = I entered the trip in the calendar.
upisati se (u nešto) – to enroll, sign up (in/for something)
- Upisao sam se na tečaj hrvatskoga.
  = I enrolled in a Croatian course.

In your sentence, we are not enrolling ourselves; we are entering an event into a calendar.
So:

No se
The object is izlet (trip), and u kalendar tells us where we entered it.

What tense is upisao sam and why do we need two words to say “I entered”?

Upisao sam is the past tense (čisti perfekt) in Croatian.

Structure:

Past participle of the main verb: upisao
Present tense of biti (to be) as auxiliary: sam (I am)

Together:

upisao sam = literally I am entered → functionally I entered / I have entered

This pattern is used for most verbs in the past tense:

napisao sam – I wrote
vidjela sam – I (female) saw
kupili smo – we bought

The auxiliary biti (sam, si, je, smo, ste, su) is required in standard past tense forms (except in some informal/colloquial omissions).

Why is it upisao and not upisala if the speaker might be female?

The past participle agrees with the gender and number of the subject.

upisao sam – masculine singular I (a man speaking)
upisala sam – feminine singular I (a woman speaking)

Your sentence:

Upisao sam u kalendar…
implies that the speaker is male.

If a woman said it, it would be:

Upisala sam u kalendar izlet na jezero na sjeveru, ali oluja je promijenila plan.

Everything else in the sentence stays the same.

Why is it u kalendar and not na kalendar?

In Croatian, the preposition often tells you the spatial relationship more precisely than in English.

u
- accusative → into / in(to) something enclosed or treated as a container:
- u kalendar – into the calendar (as an entry)
- u bilježnicu – into a notebook
- u računalo – into the computer
na
- accusative → onto a surface, or “on” in a more general/surface sense:
- na stol – onto the table
- na zid – onto the wall
- na kalendar usually suggests onto the physical surface of the calendar (e.g. sticking something on it).

Here we are entering information inside the calendar system (as an item), so u kalendar is natural.

Why is it izlet na jezero and not izlet do jezera or izlet na jezeru?

All three combinations can exist, but they mean different things.

izlet na jezero – trip to the lake
- na
  - accusative (jezero) = movement to / onto a place.
    This is what we usually say for planning a trip whose destination is the lake.
izlet do jezera – trip up to/as far as the lake
- do emphasizes the endpoint or limit.
  Sounds a bit more technical or route-focused: the trip goes as far as the lake.
izlet na jezeru – trip at/on the lake
- na
  - locative = location: being on/at the lake.
    This sounds more like talking about what happens while you’re at the lake, not the destination itself.

In your sentence, the idea is “I scheduled a trip to the lake”, so izlet na jezero with na + accusative is the most natural.

Why is it na jezero but na sjeveru? Why different cases for jezero and sjeveru?

Because one phrase expresses movement to a place, and the other expresses location at a place.

na jezero → na
- accusative (jezero)
  = to the lake (movement, destination)
na sjeveru → na
- locative (sjeveru)
  = in the north (location)

Rule with na:

na
- accusative → movement (to/onto somewhere)
na
- locative → location (on/at/in somewhere)

So:

Idemo na jezero. – We are going to the lake.
Kampiramo na jezeru. – We are camping at/on the lake.
Grad je na sjeveru. – The city is in the north.

In your sentence, izlet na jezero na sjeveru = a trip to the lake (which is) in the north.

How do we know that na sjeveru describes the lake and not the trip or the calendar?

By typical word order and meaning.

Phrase structure here is:

izlet [na jezero na sjeveru]

Croatian often places modifiers right after the noun they modify, in a chain:

izlet (trip)
→ more specific: na jezero (to the lake)
→ even more specific: na sjeveru (in the north; describing that lake or that general area)

It’s most natural to understand:

izlet (na jezero na sjeveru)
rather than
(izlet na jezero) na sjeveru or
izlet na jezero (upisan) na sjeveru (u kalendar) – which would be strange.

Contextually, na sjeveru almost always belongs to a geographical place, so it links best with jezero (lake), not with kalendar or izlet.

Why is oluja followed by je promijenila, but ja was implied in upisao sam? Why not oluja promijenila je?

Two separate things are happening:

Subject pronoun dropping
- Croatian usually omits the subject pronoun (ja, ti, on, ona, …) because the verb form (or participle + auxiliary) already shows who the subject is.
- Upisao sam… → The -o
  - sam tells you it’s I (masculine). No need for ja.
- For oluja, we can’t “drop” oluja, because it’s a noun, not a pronoun; it must be stated to know what the storm did.
Clitic position again
- In oluja je promijenila plan, je is the auxiliary for the past tense of promijeniti.
- It wants to be in the second position in the clause:
  - [Oluja] [je] [promijenila plan].
- Oluja promijenila je plan is non‑standard / wrong in normal prose; the clitic je is pushed too far to the end.

So:

Upisao sam… – correct past tense for “I entered…”
Oluja je promijenila plan. – correct past tense for “The storm changed the plan.”

Why is promijenila feminine? What is it agreeing with?

Promijenila is the past participle of promijeniti in the feminine singular form.

It agrees with the subject:

Subject: oluja (storm) – a feminine noun (like ona)
Past participle: promijenila – feminine singular

If the subject were masculine singular, you would say:

vjetar je promijenio plan – vjetar (wind, masc.) → promijenio If the subject were neuter singular:
vrijeme je promijenilo plan – vrijeme (weather, neuter) → promijenilo

So:

oluja je promijenila plan – oluja (feminine) → promijenila

Why doesn’t plan change form at the end? How do we know it’s an object?

Plan is a masculine inanimate noun with the same form in nominative singular and accusative singular:

nominative sg.: plan
accusative sg.: plan

In Oluja je promijenila plan:

oluja is the subject (nominative feminine)
promijenila is the verb (past)
plan is the direct object (accusative masculine inanimate)

We know plan is the object because:

The verb promijeniti normally takes a direct object (you change something).
The word order Subject–Verb–Object is very common and natural here.
oluja is clearly nominative (feminine -a ending), while plan fits the role of object in context.

So even though plan hasn’t visibly changed, its function in the sentence is accusative object.

What is the difference between promijenila je plan and je promijenila plan?

Oluja je promijenila plan. – correct, neutral
Oluja promijenila je plan. – ungrammatical / very odd in standard Croatian.

Reason: je is a clitic and prefers the second position in the clause:

[Oluja] [je] [promijenila plan].

You can move other elements for emphasis, but je will still try to be second:

Plan je oluja promijenila. (emphasis on plan)
– still: [Plan] [je] [oluja promijenila].

Putting je at the end (promijenila je plan) breaks this rule in a neutral sentence.

The pattern is the same with sam, si, je, smo, ste, su used as auxiliaries.

Why is it ali and not nego or a?

These conjunctions express slightly different relations:

ali – but, expressing contrast or opposition to what was expected:
- Upisao sam izlet…, ali oluja je promijenila plan. → I planned it, but a storm changed things.
a – often and/but with a weaker contrast, or just a shift to another subject:
- Upisao sam izlet u kalendar, a ti još nisi. → I entered the trip in the calendar, and/but you still haven’t.
nego – typically but rather / but instead, used after a negation:
- Nisam upisao izlet, nego sastanak. → I didn’t enter a trip, but rather a meeting.

In your sentence we have a simple, strong contrast (planned vs. disrupted by storm), so ali is the natural choice.