Learn Hardcore Greek: Λίγη καθημερινή εξάσκηση στα ελληνικά αξίζει περισσότερο από ένα μεγάλο μάθημα μία φορά. - A little daily practice in Greek is worth more than one big lesson one time.

Λίγη is the feminine form of the adjective/pronoun λίγος, -η, -ο meaning “a little / a small amount (of something uncountable)”.
It must agree in gender, number, and case with εξάσκηση, which is feminine singular, so we use λίγη (fem. nom. sg.).

λίγη εξάσκηση = a little practice (some practice, not much)
λίγο is the neuter form; you’d use it with neuter nouns or adverbially: λίγο νερό, μιλάω λίγο.
μικρή εξάσκηση would sound like “small practice” (size) rather than “a small amount of practice”, so λίγη is the natural choice here.

What does καθημερινή do in this sentence, and why does it end in -ή?

Καθημερινή is an adjective meaning “daily”. It describes εξάσκηση (“practice”), so it has to agree with it:

εξάσκηση: feminine, singular, nominative
καθημερινή: feminine, singular, nominative to match

So λίγη καθημερινή εξάσκηση literally means “a little daily practice”.
You could also say λίγη εξάσκηση κάθε μέρα (“a little practice every day”) with the same basic meaning; καθημερινή just packs that idea into a single adjective.

What does εξάσκηση mean exactly, and how is it different from άσκηση or πρακτική?

Εξάσκηση means “practice (the activity of practising something)”, especially in the sense of training or drilling a skill (language, music, sport, etc.).

εξάσκηση στα ελληνικά = practice in Greek (using the language)
άσκηση is more like “exercise” (a specific task / exercise in a book, or physical exercise).
πρακτική is often “practical training / internship” (e.g. κάνω πρακτική σε μια εταιρεία).

In this sentence, εξάσκηση is the natural word, because we’re talking about regularly practising the language, not doing one exercise or doing an internship.

Why is it στα ελληνικά and not something like σε ελληνικά or στην ελληνική γλώσσα?

Στα is the contraction of σε + τα (preposition + definite article):

σε + τα ελληνικά → στα ελληνικά

Τα ελληνικά is the standard way to say “(the) Greek (language)” in modern Greek (neuter plural).
So στα ελληνικά literally means “in the Greek (language)”, i.e. “in Greek.”

You could say στην ελληνική γλώσσα (“in the Greek language”), but that sounds more formal and less natural in everyday speech than στα ελληνικά.

What is the subject of αξίζει? Is this verb used impersonally here?

Αξίζει comes from αξίζω, meaning “to be worth”.
Grammatically, the subject here is λίγη καθημερινή εξάσκηση στα ελληνικά:

Λίγη καθημερινή εξάσκηση στα ελληνικά (subject) αξίζει (verb) περισσότερο…

So it is not impersonal in this sentence; it’s a normal 3rd person singular verb with a subject.
However, αξίζει can also be used impersonally in other contexts, like:

Αξίζει να προσπαθήσεις. = It’s worth trying.

Why is περισσότερο used here, and not πιο πολύ? What’s the difference?

Both περισσότερο and πιο πολύ can mean “more” quantitatively, and both would be understood:

…αξίζει περισσότερο από…
…αξίζει πιο πολύ από…

Here περισσότερο functions as an adverb meaning “to a greater degree / more” and sounds slightly more neutral or standard, especially in written style.
Πιο πολύ is a bit more colloquial in tone. In everyday speech, many speakers would naturally say πιο πολύ, but περισσότερο is perfectly natural and maybe a shade more “careful” Greek.

What is the role of από in περισσότερο από ένα μεγάλο μάθημα? Does it affect the case?

Από is used here to introduce the second element in a comparison (the thing you’re comparing with):

αξίζει περισσότερο από ένα μεγάλο μάθημα
= it is worth more than one big lesson

In this usage, από is followed by the normal case you’d expect (here: ένα μεγάλο μάθημα, accusative, as a plain noun phrase).
So the pattern is:
[comparative] + από + [thing it is compared to]
e.g. είναι καλύτερο από αυτό, τρέχω πιο γρήγορα από εσένα.

Why is it ένα μεγάλο μάθημα (neuter) and not masculine or feminine? What does μάθημα imply here?

Μάθημα is a neuter noun in Greek (το μάθημα), so its article and adjective must also be neuter:

ένα (neuter article)
μεγάλο (neuter adjective)
μάθημα (neuter noun)

So ένα μεγάλο μάθημα = “one big lesson / a single large lesson” (perhaps long or intensive).
The sentence contrasts a little daily practice with one big occasional lesson in terms of value.

Why is μία φορά used, and does it literally mean “one time”? Can I also say μια φορά?

Yes, μία φορά literally means “one time”, and in this context it corresponds to “once”:

…από ένα μεγάλο μάθημα μία φορά.
= “…than one big lesson once.”

In everyday Greek, μία is very often pronounced and written μια:

ένα μεγάλο μάθημα μια φορά – extremely common, same meaning.

So μία φορά and μια φορά are both correct here; μια is just the more colloquial/contracted form of μία.

Why is there no article before λίγη καθημερινή εξάσκηση? Could it be λίγη η καθημερινή εξάσκηση?

Mass/uncountable nouns in Greek (like εξάσκηση, νερό, δουλειά) often appear without an article when talking about them in a general, indefinite way:

λίγη εξάσκηση, πολύ δουλειά, λίγο νερό

Here we’re talking about “a bit of daily practice” in general, not some specific, identified practice, so we leave out the article.

Λίγη η καθημερινή εξάσκηση is grammatically possible but changes the meaning and tone; it would sound like “The daily practice is little” (commenting about an already-known, specific daily practice), which is not what this sentence is doing.

Could we say λίγη εξάσκηση στα ελληνικά κάθε μέρα instead of λίγη καθημερινή εξάσκηση στα ελληνικά? Is there any nuance difference?

Yes, λίγη εξάσκηση στα ελληνικά κάθε μέρα is perfectly natural and very common.
Meaning-wise it is almost the same:

λίγη καθημερινή εξάσκηση στα ελληνικά
λίγη εξάσκηση στα ελληνικά κάθε μέρα

The first packs “daily” into the adjective καθημερινή; the second expresses it adverbially with κάθε μέρα (“every day”).
If anything, κάθε μέρα feels a bit more colloquial and explicit; καθημερινή sounds slightly more “compressed” and a touch more formal, but the difference is small.

Can we say λίγη εξάσκηση στα ελληνικά καθημερινά instead of using καθημερινή? How do καθημερινή and καθημερινά differ?

Yes, λίγη εξάσκηση στα ελληνικά καθημερινά is also correct and natural. Here:

καθημερινή is an adjective: καθημερινή εξάσκηση = “daily practice.”
καθημερινά is an adverb: it means “daily / every day,” modifying the verb or the whole situation.

So you could say, for example:

Κάνω λίγη εξάσκηση στα ελληνικά καθημερινά.
= I do a little practice in Greek every day.

You wouldn’t say λίγη καθημερινά εξάσκηση; the position and form would be wrong. Use καθημερινή before the noun or καθημερινά as a separate adverbial word.