Learn Hardcore Croatian: Na mreži sam našla oglas za besplatan tečaj hrvatskog jezika. - On the internet I found an ad for a free Croatian language course.

na internetu = on the internet (very common)
na netu = on the net (more colloquial)
online = a direct borrowing from English, used but somewhat more in informal speech or marketing.

So:

Na mreži sam našla oglas… ≈ Na internetu sam našla oglas… ≈ “I found an ad online …”

What form is “mreži” and why isn’t it just “mreža”?

mreži is the locative singular form of mreža.

Base form (dictionary form): mreža (feminine noun, nominative singular)
After the preposition na meaning “on, in” in the sense of location, Croatian usually uses the locative case.

So:

na mreži = on the net (locative)
mreža by itself is nominative (“the net” as subject), but here we need locative because of the preposition na.

Why is the past tense split into “sam našla” instead of just one word like in English “found”?

Croatian forms the ordinary past tense with two parts:

The present tense of biti (to be) as an auxiliary:
- ja sam
- ti si
- on/ona/ono je
- mi smo
- vi ste
- oni/one/ona su
The l-participle (past participle-like form) of the main verb, which agrees in gender and number:
- naći → našao (masc. sg.), našla (fem. sg.), našli (masc. pl.), etc.

So:

(Ja) sam našla = I (female) found
A man would say: (Ja) sam našao = I (male) found

English just changes the verb form (find → found). Croatian keeps a present form of “to be” + a special past form of the main verb.

How would a man say this sentence instead of a woman?

Only the past participle changes to match the speaker’s gender:

Original (female speaker):
Na mreži sam našla oglas za besplatan tečaj hrvatskog jezika.
Male speaker:
Na mreži sam našao oglas za besplatan tečaj hrvatskog jezika.

Everything else is the same. The auxiliary sam doesn’t change; only našla → našao.

Why isn’t “ja” (I) used? Could I say “Ja sam našla…”?

In Croatian, subject pronouns like ja, ti, on, ona are usually dropped, because the verb already shows:

person (I, you, he, etc.) through the auxiliary (sam, si, je…)
and in the past tense, often gender (našao / našla)

So:

Sam našla oglas… is not idiomatic (you need something before sam)
Ja sam našla oglas… is correct
Našla sam oglas… is also correct
Na mreži sam našla oglas… is what you have.

Croatians generally omit the subject pronoun unless they want to emphasize it:

Ja sam našla oglas, a ne ti. = I found the ad, not you.

Why is the little word “sam” in the middle: “Na mreži sam našla…”? Can I put it somewhere else?

Forms of biti used as auxiliaries (sam, si, je, smo, ste, su) are clitics: they like to stand in second position in the clause.

In your sentence:

1st “chunk”: Na mreži
2nd position: sam
then the rest: našla oglas za besplatan tečaj hrvatskog jezika

Other natural orders:

Našla sam oglas na mreži.
Na mreži sam našla oglas. (your version)

You cannot just move sam anywhere you want; putting it in second position (after the first word or phrase) is the default rule in Croatian.

What exactly does “oglas” mean? Is it the same as “advertisement”?

Yes. oglas = ad, advertisement, notice.

Typical uses:

oglas u novinama = a newspaper ad
internetski oglas = an online ad
mali oglasi = classified ads

A common synonym is reklama, but:

reklama often suggests a commercial or TV ad.
oglas is more neutral for any kind of announcement/advert, including a simple notice or listing.

In your sentence, oglas is accusative singular masculine, but it looks the same as the base form:

Nominative: oglas – “the ad” (subject)
Accusative: oglas – “(I found) an ad” (object)

Why is it “za besplatan tečaj”? What case is used after “za”?

The preposition za (“for”) normally takes the accusative case.

za
- accusative → goal, purpose: “for, in order to get”

Here:

tečaj (course) is a masculine noun.
In the accusative singular (inanimate), it looks the same as nominative: tečaj.
The adjective besplatan also appears in its masculine singular accusative (inanimate) form, which equals its nominative form.

So:

besplatan tečaj = a free (of charge) course (accusative, object of za)

I’ve seen “besplatan tečaj” and “besplatni tečaj”. What’s the difference?

Both are possible; the difference is subtle and mostly stylistic.

besplatan tečaj – very common in modern standard usage
besplatni tečaj – also correct; can sound a bit more formal or old-fashioned in some contexts

Grammatically, both are masculine singular forms of “free (of charge)”. They are just two different patterns within the same adjective paradigm. In practice, you can use “besplatan tečaj” safely and you’ll sound natural.

What does “tečaj” mean exactly, and how is it different from other words like sat, razred, or lekcija?

tečaj is closest to English “course”:

tečaj hrvatskog jezika = Croatian language course
tečaj kuhanja = cooking course

Other related words:

sat – literally “hour”; also used for a class period (“imati sat hrvatskog” = to have a Croatian class/lesson)
razred – “class” in the sense of school year/group (e.g., 3rd grade class)
lekcija – “lesson” (content unit), more like the chapter of a textbook or topic of study.

So tečaj is the whole organized course, usually spanning weeks or months.

Why is it “hrvatskog jezika” and not just “hrvatski jezik” or “hrvatski”?

tečaj is being specified: a course of what? → of Croatian language.
In Croatian, this relationship is normally expressed with the genitive case:

tečaj čega? = a course of what?
tečaj hrvatskog jezika = course of Croatian language

So:

hrvatskog is genitive singular masculine of the adjective hrvatski (Croatian).
jezika is genitive singular masculine of jezik (language).

If you said hrvatski jezik (nominative), that would be fine by itself (“the Croatian language”), but after “tečaj” we expect a genitive:

✗ tečaj hrvatski jezik – ungrammatical
✓ tečaj hrvatskog jezika – grammatical and natural

In everyday speech, people often omit “jezika” if it’s clear:

tečaj hrvatskog – a Croatian (language) course

What do the endings “-og” in “hrvatskog” and “-a” in “jezika” tell me?

They show case, gender, and number:

hrvatskog
- from hrvatski (Croatian)
- genitive singular masculine
- common endings: -og or -oga (both correct: hrvatskog / hrvatskoga)
jezika
- from jezik (language, tongue)
- genitive singular masculine
- many masculine nouns have -a in the genitive:
  - grad → grada (city → of the city)
  - jezik → jezika (language → of the language)

Together hrvatskog jezika = “of the Croatian language” (genitive phrase).

Is “tečaj hrvatskog” without “jezika” also correct?

Yes, very common in real speech.

Full form: tečaj hrvatskog jezika
Shorter, but fully acceptable: tečaj hrvatskog

The noun jezik (language) is understood from context, so it can be dropped. This happens often with language courses:

tečaj engleskog (jezika)
tečaj njemačkog (jezika)

How do you pronounce “mreži”, “tečaj”, and “hrvatskog jezika”?

Approximate guidance (not IPA):

mreži – MREH-zhee
- ž like the s in “measure”
- stress usually on mre-
tečaj – TEH-chai
- č like ch in “church”
- aj like eye
hrvatskog jezika – roughly HR-vats-kog YEH-zi-ka
- h is pronounced (a bit like in “Bach” in some dialects, but you can use English h)
- r is rolled or tapped
- j always like English y in “yes” (so je = “yeh”)

The sentence as a whole flows as:

Na MRE-ži sam NAŠ-la O-glas za BES-pla-tan TEH-chai HR-vats-kog YEH-zi-ka.