S njim učim hrvatski svaki dan.

Croatian often leaves out subject pronouns because the verb ending already shows who is doing the action.

Here, učim means I learn / I am learning / I study, so ja is not necessary.

• Učim = I learn
• Ja učim = I learn, but with extra emphasis on I

You would usually add ja only if you want contrast or emphasis, such as Ja učim, a on radi.

Because s means with, and this preposition requires the instrumental case.

The pronoun on changes form in different cases:

• on = he
• njega / ga = him
• njemu / mu = to him
• njim = with him / by him

So:

• s njim = with him

Njim is the instrumental singular form of on.

After the preposition s meaning with, Croatian uses the instrumental:

• s prijateljem = with a friend
• s učiteljem = with the teacher
• s njim = with him

This is one of the most common uses of the instrumental case in Croatian.

It can mean all of those, depending on context.

Croatian uses the simple present tense here, and English may translate it in different ways:

• I learn Croatian with him every day
• I am learning Croatian with him every day
• I study Croatian with him every day

All are possible translations of S njim učim hrvatski svaki dan.

Yes, sometimes.

This verb can mean learn or teach, depending on the structure of the sentence.

• Učim hrvatski = I am learning Croatian
• Učim djecu hrvatski = I teach children Croatian

In your sentence, učim hrvatski clearly means I am learning/studying Croatian, because hrvatski is the thing being learned.

Because Croatian often uses the language adjective by itself when talking about languages.

So:

• učim hrvatski = I learn Croatian
• govorim engleski = I speak English
• studira francuski = he/she studies French

This is basically a shortened form of hrvatski jezik, but in normal speech the noun jezik is often omitted.

Also, Croatian has no articles, so you do not need a word like the before it.

In Croatian, names of languages are written with a lowercase letter.

So you write:

• hrvatski
• engleski
• njemački

But nationality words and country names are capitalized:

• Hrvat = Croat
• Hrvatska = Croatia

So hrvatski is correct here.

Here hrvatski is functioning as the direct object of učim, so it is in the accusative.

It comes from the longer expression hrvatski jezik. Since jezik is masculine inanimate, the accusative singular looks the same as the nominative singular, so the form stays hrvatski.

That is why you see:

• učim hrvatski
• not a visibly different form

Yes, both are possible.

• svaki dan = every day
• svakog dana = every day

In your sentence, svaki dan is a very common and natural way to express repeated time. It uses the accusative form.

Svakog dana is also very common and means almost the same thing. As a learner, you should understand both.

Yes. Croatian word order is quite flexible.

Your sentence:

• S njim učim hrvatski svaki dan

is perfectly natural. It slightly highlights s njim, as if the speaker wants to stress with him.

Other possible orders include:

• Učim hrvatski s njim svaki dan
• Svaki dan učim hrvatski s njim
• Hrvatski učim s njim svaki dan

The basic meaning stays the same, but the focus can shift depending on what comes first.

In standard Croatian, s njim is the usual form.

The longer form sa is mainly used before certain sounds or clusters that are harder to pronounce, for example:

• sa mnom
• sa sobom
• sa sestrom

With njim, standard Croatian normally uses s njim.

You may hear sa njim in some regional or colloquial speech, but s njim is the safer standard form to learn.