Learn Hardcore Croatian: Na mreži pratim jedan kanal na kojem učiteljica svaki dan objavljuje kratku lekciju. - On the internet I follow one channel on which the teacher publishes a short lesson every day.

Na internetu pratim jedan kanal… – On the internet I follow a channel…
Online pratim jedan kanal… – also understandable and increasingly common, but slightly more colloquial/Anglicized.

So na mreži is just one natural option. It uses the noun mreža (network, net, web) instead of internet, but the meaning in context is “online”.

What case is mreži in, and why is it used here?

Mreži is in the locative singular (of mreža). After the preposition na, Croatian uses:

Accusative for motion onto/into something:
- Idem na internet. – I’m going onto the internet.
Locative for being on/in something (location):
- Na internetu sam. – I am on the internet.
- Na mreži pratim… – On the web I follow…

Here we’re talking about where the action happens (location), not movement toward it, so na + locative → na mreži.

Why is the verb pratim used instead of something like gledam or slijedim?

Pratiti means to follow in the sense of keeping up with content over time: subscribing, regularly checking, watching new posts, etc.

Pratim jedan kanal implies: I follow this channel and keep up with what it posts.
Gledam jedan kanal would focus more on watching it (like TV), not necessarily regularly.
Slijedim is “follow” mainly in a physical or abstract sense (follow someone’s lead, follow a path, follow rules); it’s not the usual verb for “I follow a YouTube channel”.

So pratim is the natural choice for following an online channel.

Do we really need jedan in pratim jedan kanal? What’s the difference with pratim kanal?

You can say it either way:

Pratim kanal na kojem… – I follow a channel on which…
Pratim jedan kanal na kojem… – I follow a (certain) channel on which…

Jedan here works like English “a certain / one particular” and slightly emphasizes that it’s one (specific, but not further identified) channel.

Without jedan, it’s more neutral and general. With jedan, it sounds more like “there’s this one channel that I follow…”, setting it up as new information in the conversation.

What exactly is going on with na kojem? Why that form, and what is it agreeing with?

Na kojem is a relative phrase (preposition + relative pronoun) meaning on which.

The relative pronoun is koji (who/which/that).
It must agree with its antecedent (kanal) in gender and number, and it takes the case required by the preposition na:
- kanal – masculine singular
- na (location) → locative
- masculine singular locative of koji = kojem

So you get:

kanal na kojem… – a channel on which…

If the noun were feminine or neuter, you’d see different forms:

stranica na kojoj… – a page on which… (fem.)
mjesto na kojem… – a place at which… (neut., same form as masc. in locative).

Could we use gdje instead of na kojem? What’s the difference?

Yes, you could say:

…pratim jedan kanal gdje učiteljica svaki dan objavljuje kratku lekciju.

Native speakers do say this, especially in speech.

Differences:

na kojem is more precise (literally “on which (channel)”), and more typical in careful or written language.
gdje literally means “where” and is a bit looser/colloquial here.

Both are acceptable; na kojem sounds slightly more formal and grammatically explicit.

Why is it učiteljica and not učitelj? Does the form change the meaning?

Yes. Croatian regularly marks grammatical gender for professions:

učitelj – male teacher
učiteljica – female teacher

Using učiteljica clearly tells you the teacher is a woman. If the teacher were male, the natural form would be:

…kanal na kojem učitelj svaki dan…

Unlike in English, it’s less common (though possible) to use the masculine form generically for “a teacher” when the person is known and specifically female; people usually match the actual gender.

Why is kratku lekciju in that exact form? What case and gender is it?

Kratku lekciju is accusative singular feminine, because it’s the direct object of objavljuje:

Verb: objavljuje – she publishes
Object: (što?) kratku lekciju – what? a short lesson

Details:

lekcija is feminine: nominative lekcija, accusative lekciju
kratak (masc.) → kratka (fem. nominative) → kratku (fem. accusative)

So the adjective and noun both take feminine singular accusative:

kratka lekcija (subject, nominative)
kratku lekciju (object, accusative)

Why is it objavljuje and not objavi? What’s the aspect here?

Croatian distinguishes imperfective and perfective aspect:

objavljivati (imperfective) → present: objavljuje
objaviti (perfective) → future/once‑off actions (its “present” form usually refers to future).

We use the imperfective aspect for repeated, habitual actions:

učiteljica svaki dan objavljuje kratku lekciju – she publishes every day, routinely.

If you said:

učiteljica će sutra objaviti kratku lekciju – she will publish a (single) short lesson tomorrow.

So objavljuje fits the meaning “does this regularly”.

Why is the present tense used in svaki dan objavljuje to describe a habitual action?

In Croatian, the simple present is used both for:

actions happening now, and
regular / habitual actions.

So:

Svaki dan objavljuje kratku lekciju. – She publishes a short lesson every day.
Često čitam. – I often read.

There’s no separate tense like English “does publish”; the plain present objavljuje covers everyday routines by itself, especially when you add an adverbial like svaki dan.

Can the word order of svaki dan change? For example, is učiteljica svaki dan objavljuje kratku lekciju the only option?

Word order in Croatian is relatively flexible, so you can move svaki dan:

Na mreži pratim jedan kanal na kojem učiteljica svaki dan objavljuje kratku lekciju.
…na kojem učiteljica objavljuje svaki dan kratku lekciju.
…na kojem učiteljica objavljuje kratku lekciju svaki dan.

All are grammatical. The default, neutral-sounding choice is often like the original sentence, where svaki dan comes right before the verb. Other positions may slightly emphasize svaki dan or the object, but the basic meaning stays the same.

Should there be a comma before na kojem in Croatian, like in English “a channel on which…”?

No comma is needed here in Croatian.

Croatian generally does not use a comma before a restrictive relative clause (one that specifies which channel):

Pratim jedan kanal na kojem učiteljica… – no comma.

A comma is used with non‑restrictive (extra information) clauses, which is not the case here. So the spelling in the given sentence is correct.

Why is lekciju singular when she publishes something every day? Could it be plural?

Lekciju is singular because each day she publishes one short lesson:

svaki dan → each individual day
kratku lekciju → one lesson per day

If she published several lessons per day, you would use the plural:

…svaki dan objavljuje kratke lekcije. – she publishes short lessons (several) every day.

So the singular fits the idea of “one per day”.