Danas učim više nego jučer.

Because Croatian usually drops subject pronouns when the verb ending already makes the subject clear.

• učim = I learn / I am learning
• the ending -m shows 1st person singular

So Danas učim više nego jučer is perfectly complete without ja.

You can add ja only for emphasis or contrast:

• Ja danas učim više nego jučer.
• Ja učim više, a on manje.

It can mean both, depending on context.

Croatian does not have a separate continuous form like English am learning. The ordinary present tense often covers both:

• I learn
• I am learning

Here, because of danas (today), the sentence is naturally understood as something like I am learning more today than yesterday.

Because Croatian does not normally build the present progressive the way English does.

English:

• I am learning

Croatian:

• učim

So Croatian uses the simple present form učim, not a structure with to be + -ing.

Učim is the 1st person singular present tense of the verb učiti.

So:

• učiti = the verb in its dictionary form
• učim = I learn / I am learning

This verb is imperfective, which fits an ongoing or repeated activity.

Because Croatian distinguishes aspect.

• učiti = to learn / to study, as an ongoing process
• naučiti = to learn something completely, to master it

In this sentence, the speaker is talking about the activity of learning/studying today, so učiti is the natural choice.

Using naučim would change the meaning toward I learn/master something completely, not simply I am studying/learning.

Više means more here, and it works as a comparative adverb.

It modifies the verb učim, so the idea is:

• I learn more
• not I am a bigger learner or anything like that

A useful way to see it:

• učim = I learn
• učim više = I learn more

Nego introduces the comparison: than.

So:

• više nego jučer = more than yesterday

This is the normal way to compare with something like an adverb or a whole implied idea.

In fact, više nego jučer is understood as something like:

• more than (I did) yesterday

A very important point for learners:

• od jučer usually means since yesterday, not than yesterday

So in this sentence, nego jučer is the correct and natural choice.

Because jučer is an adverb, not a noun.

Adverbs like jučer (yesterday) are generally indeclinable, which means they do not change for case, gender, or number.

The same is true for danas (today).

So you just use:

• danas
• jučer

with no case endings added.

Yes. Croatian word order is quite flexible.

The given sentence:

• Danas učim više nego jučer.

is a very natural, neutral order.

But other orders are possible, depending on emphasis, for example:

• Učim više danas nego jučer.
• Više učim danas nego jučer.

These alternatives can sound slightly different in focus, but the core meaning stays the same.

You can leave it out if the context already makes today clear.

• Učim više nego jučer.

still works and means I’m learning more than yesterday.

Adding danas makes the time contrast more explicit:

• today versus yesterday

So it is not grammatically required, but it is very useful for clarity and emphasis.