Στον πληθυντικό η λέξη αλλάζει λίγο.

Στον is the common contracted form of σε τον.

• σε = in / at / to
• τον = the (masculine accusative singular)

So στον πληθυντικό literally means in the plural.

In modern Greek, σε + άρθρο is very often written as one word:

• σε + τον = στον
• σε + τη(ν) = στην
• σε + το = στο

This is extremely common and worth recognizing right away.

Because after σε / στον, Greek normally uses the accusative case.

The dictionary form is:

• ο πληθυντικός = the plural

But in this sentence it becomes:

• στον πληθυντικό

So you are seeing the masculine accusative singular form of πληθυντικός.

This expression is idiomatic:

• στον ενικό = in the singular
• στον πληθυντικό = in the plural

η λέξη means the word.

Greek nouns have grammatical gender, and λέξη happens to be feminine. That is why it takes the feminine article η.

So:

• η λέξη = the word
• της λέξης = of the word
• τη λέξη = the word (object form)

The gender here is grammatical, not logical. You simply learn λέξη together with its article: η λέξη.

Because the subject is η λέξη — the word — and that is singular.

αλλάζει is the 3rd person singular form of αλλάζω (to change).

So:

• η λέξη αλλάζει = the word changes

Compare:

• η λέξη αλλάζει = the word changes
• οι λέξεις αλλάζουν = the words change

The verb agrees with the subject in number.

αλλάζει is the present tense, 3rd person singular of αλλάζω.

Basic pattern:

• αλλάζω = I change
• αλλάζεις = you change
• αλλάζει = he/she/it changes

In this sentence, it means changes or does change.

Greek often uses the present tense in a straightforward way here, just like English:

• The word changes a little.

Here λίγο is being used as an adverb, not as an adjective.

It modifies the verb αλλάζει:

• αλλάζει λίγο = changes a little

When λίγος / λίγη / λίγο means little / a small amount of as an adjective, it changes form to match the noun. But when it means a little and modifies a verb, the fixed adverbial form λίγο is used.

Compare:

• λίγο νερό = a little water
• αλλάζει λίγο = it changes a little

Yes, Greek word order is more flexible than English word order.

This sentence begins with Στον πληθυντικό to set the context first:
In the plural, the word changes a little.

That sounds very natural in Greek.

Other possible orders are also grammatical, for example:

• Η λέξη αλλάζει λίγο στον πληθυντικό.
• Η λέξη στον πληθυντικό αλλάζει λίγο.

But the original version is very natural if the speaker wants to emphasize what happens in the plural.

Yes, very often it is understood as something like:

• στον πληθυντικό αριθμό = in the plural number

But Greek commonly just says:

• στον πληθυντικό

in grammar explanations, and that is completely normal.

The same happens with:

• στον ενικό = in the singular

So learners should treat στον πληθυντικό as a standard grammar phrase.

A simple pronunciation guide is:

Ston plithindikó i léxi allázi lígo.

A few useful notes:

• Στον = ston
• πληθυντικό has the stress on the last syllable: plithindikÓ
• λέξη has stress on LE
• αλλάζει has stress on LA
• λίγο has stress on LI

Also note:

• η is pronounced ee
• ξ in λέξη sounds like ks

It means changes, not is changed.

Greek is using the active verb naturally:

• η λέξη αλλάζει λίγο = the word changes a little

English sometimes says similar things in grammar explanations:

• the word changes
• the form changes

So this is a normal and direct way to express the idea in Greek.