Learn Hardcore Croatian: Ponekad mu pišem kratka pisma samo da mu kažem koliko mi je važna naša ljubav. - Sometimes I write him short letters just to tell him how important our love is to me.

Ponekad mu pišem kratka pisma – mu = to him, indirect object of pišem (I write to him).
samo da mu kažem – mu = to him, indirect object of kažem (to tell him).

So it’s basically:
I sometimes write him short letters just to tell him how important our love is to me.
Both verbs (pišem, kažem) need their “to him” expressed, so mu appears twice.

What case is mu, and what does that case express?

Mu is the unstressed (clitic) form of the dative pronoun njemu (3rd person singular, masculine/neuter).

Case: dative
Typical meaning: “to/for him/it”

In this sentence:

mu pišem = I write to him
da mu kažem = (so that) I tell him

The dative often marks the person who receives something (letters, information, help, etc.).

Can I say Ponekad pišem mu kratka pisma instead of Ponekad mu pišem kratka pisma?

In standard Croatian, that word order is wrong or at least very unnatural.

Mu is a clitic pronoun, and clitics normally go in “second position” in the clause – they should come right after the first stressed word or phrase.

So:

✅ Ponekad mu pišem kratka pisma.
✅ Ja mu ponekad pišem kratka pisma.
✅ Kratka mu pisma ponekad pišem. (more marked, but grammatically OK)
❌ Ponekad pišem mu kratka pisma. (clitic too far from the beginning)

Keep mu as close to the beginning (second position) as the rules allow.

Could I leave out the pronouns and just say:
Ponekad pišem kratka pisma samo da kažem koliko mi je važna naša ljubav?

You can, and the sentence is grammatically correct, but the meaning changes:

Original: very clearly to him:
I sometimes write him short letters just to tell him…
Without mu:
I sometimes write short letters just to say how important our love is to me.
(to no one in particular / in general / maybe publish them, etc.)

So if the idea is that you are writing to a specific person, keep mu with both verbs. Removing mu makes the sentence vaguer and less personal.

Why is it kratka pisma, not kratke pisma?

Because pismo is a neuter noun:

singular: pismo (letter)
plural: pisma (letters)

Adjectives must agree in gender, number, and case with the noun:

nominative/accusative neuter singular: kratko pismo
nominative/accusative neuter plural: kratka pisma

So kratka pisma is the correct neuter plural form.
Kratke pisma would be wrong, because kratke is feminine plural (or genitive sg. fem.), not neuter.

What case are kratka pisma, and why?

Kratka pisma is in the accusative plural:

It’s the direct object of pišem (I write what? short letters).
For neuter nouns, nominative plural and accusative plural look the same (pisma).
That’s why it visually looks like nominative, but its function in the sentence is accusative.

So grammatically:
pišem (verb) + što? (what?) → kratka pisma (accusative plural, neuter).

Why is pišem in the present tense if I’m talking about something I do from time to time?

In Croatian, the present tense of an imperfective verb is used for:

actions happening right now
habits / repeated actions

Pisati – pišem is imperfective, so:

Ponekad mu pišem kratka pisma.
= Sometimes I write him short letters (this is a habit / something I do from time to time).

This is very similar to English “I sometimes write him letters,” which is also present tense used for habits.

Why do we use da mu kažem and not an infinitive like samo reći mu?

Croatian usually does not use the infinitive to express purpose the way English does (“to tell him,” “in order to tell him”).

Instead, it uses a “da”‑clause with a finite verb (present tense):

samo da mu kažem = just (so that) I tell him / just to tell him

Using an infinitive here (samo reći mu) is not standard Croatian; it sounds foreign-influenced (English, German, etc.). Native speakers overwhelmingly prefer:

samo da mu kažem
or, more formal: samo kako bih mu rekla/rekla (with conditional), but that changes style and nuance.

What exactly does samo da express here? Is it “just that” or “just so that”?

Here samo da means “just so that / only in order to” and introduces a purpose clause:

pišem kratka pisma samo da mu kažem…
= I write short letters only in order to tell him…
= I write short letters just so I can tell him…

So it’s not emphasizing content (“just that I tell him, not more”) as much as purpose: the only reason for writing is to tell him this thing.

How does koliko mi je važna naša ljubav work grammatically?

Breakdown:

koliko – “how (much)” / “how” (degree)
mi – dative of ja (“to me”) – an indirect experiencer
je – 3rd person singular of biti (to be)
važna – adjective “important”, feminine singular
naša ljubav – “our love”, feminine singular

Literally:

koliko mi je važna naša ljubav
= how (much) to-me is important our love
= how important our love is to me

Note that Croatian often puts the subject at the end in such clauses:
koliko mi je važna naša ljubav, instead of the more “textbook” order koliko je naša ljubav važna meni. Both are possible, but the given one is more natural and focuses on “our love” at the end.

Why is it važna and naša ljubav, not važan and naš ljubav?

Because ljubav (love) is grammatically feminine in Croatian.

Adjectives must agree with the noun they describe:

gender: feminine
number: singular
case: nominative (here, as subject complement)

So you need feminine singular forms:

važna (fem. sg.) – important
naša (fem. sg.) – our

Therefore: važna naša ljubav, not važan naš ljubav.

Why is it naša ljubav (nominative) and not našu ljubav (accusative)?

In the phrase koliko mi je važna naša ljubav:

naša ljubav is the subject of the verb je (“is”).
As a subject, it must be in the nominative case.

The structure is essentially:

Naša ljubav je važna (meni).
→ Our love is important (to me).

When you turn it into a “how” clause with koliko, the subject stays nominative:

koliko mi je važna naša ljubav
not
❌ koliko mi je važna našu ljubav

Našu ljubav (accusative) would be used if love were the object, e.g.:

Koliko volim našu ljubav. – How much I love our love.

Could I drop mi in koliko mi je važna naša ljubav?

Yes, you can say:

koliko je važna naša ljubav – how important our love is

But:

With mi: koliko mi je važna naša ljubav
→ explicitly: how important our love is to me
Without mi: koliko je važna naša ljubav
→ more general: how important our love is (in general / objectively)

The version with mi makes the sentence more personal and emotional, highlighting your perspective.

If I’m writing to a woman, should it still be mu, or should it be joj?

For a woman, you should use joj, not mu.

Dative pronouns:

mu = to him / to it (masculine or neuter)
joj = to her (feminine)

So if the person is female, you would say:

Ponekad joj pišem kratka pisma samo da joj kažem koliko mi je važna naša ljubav.
= Sometimes I write her short letters just to tell her how important our love is to me.

What is the difference between kažem and govorim here? Could I say samo da mu govorim?

Reći/kazati – kažem and govoriti – govorim are related but not identical:

kažem (perfective reći/kazati) – to say / to tell (something specific)
→ focuses on the content of a specific statement
govorim (imperfective govoriti) – to speak / to be speaking / to talk (in general or continuously)
→ focuses on the ongoing activity of speaking

In this sentence, you want to express the purpose of writing letters: to tell him one important thing (how important our love is).

So:

✅ samo da mu kažem koliko mi je važna naša ljubav – natural and correct.
❌ samo da mu govorim koliko mi je važna naša ljubav – sounds like you want to “keep talking to him” about it all the time; it doesn’t fit well as a purpose of writing short letters.

For a specific message, kažem is the right choice.