Η κοπέλα δίπλα μου λέει ότι αυτό το αναψυκτικό είναι πολύ γλυκό.

In Greek, a demonstrative like αυτός / αυτή / αυτό normally goes together with the definite article:

• αυτό το αναψυκτικό = this soft drink
• εκείνο το αναψυκτικό = that soft drink

So Greek usually says this + the + noun.
You would not normally say αυτό αναψυκτικό.

Here:

• αυτό = this for a neuter singular noun
• το = the neuter singular definite article
• αναψυκτικό = the noun

Because αναψυκτικό is neuter singular, both αυτό and το are in neuter singular too.

δίπλα μου means next to me or beside me.

Breakdown:

• δίπλα = next to / beside
• μου = me / my, here meaning to me / of me / me in a fixed expression

So η κοπέλα δίπλα μου means the girl next to me.

This is a very common Greek way to describe location:

• δίπλα μου = next to me
• μπροστά μου = in front of me
• πίσω μου = behind me

Greek often uses a shorter structure where English might use a relative clause.

So instead of:

• η κοπέλα που είναι δίπλα μου = the girl who is next to me

Greek can simply say:

• η κοπέλα δίπλα μου = the girl next to me

Both are possible, but the shorter version is very natural and common.

In this sentence, the natural reading is that δίπλα μου describes η κοπέλα:

• Η κοπέλα δίπλα μου = The girl next to me

Because the phrase comes right after κοπέλα, it is understood as modifying that noun.

So the structure is:

• Η κοπέλα δίπλα μου = subject
• λέει = verb
• ότι... = the content of what she says

λέει is the 3rd person singular present of λέω = to say.

So:

• λέω = I say
• λες = you say
• λέει = he/she/it says

Here it matches η κοπέλα, which is third person singular:

• Η κοπέλα λέει... = The girl says...

It can mean either, depending on context.

Greek present tense often covers both:

• she says
• she is saying

So Η κοπέλα δίπλα μου λέει... could be understood as either:

• The girl next to me says...
• The girl next to me is saying...

English chooses between simple and progressive more often than Greek does.

ότι means that and introduces a content clause:

• λέει ότι... = says that...

So the sentence is structured like this:

• Η κοπέλα δίπλα μου λέει = The girl next to me says
• ότι αυτό το αναψυκτικό είναι πολύ γλυκό = that this soft drink is very sweet

In everyday Greek, ότι can sometimes be replaced by πως, and in casual speech it may even be omitted:

• λέει ότι...
• λέει πως...
• sometimes just λέει...

All are possible, though ότι is perfectly standard.

This is a very common learner question.

• ότι = that
• ό,τι = whatever / anything that

So here, the correct word is ότι because it introduces what she says:

• λέει ότι... = she says that...

But:

• Πάρε ό,τι θέλεις. = Take whatever you want.

Notice that ό,τι is written with a comma to distinguish it.

Because it agrees with αναψυκτικό, which is neuter singular.

Greek adjectives agree with the noun they describe in:

• gender
• number
• case

Here:

• αναψυκτικό = neuter singular
• γλυκό = neuter singular form of sweet

So:

• ο καφές είναι γλυκός = the coffee is sweet
• η σοκολάτα είναι γλυκιά = the chocolate is sweet
• το αναψυκτικό είναι γλυκό = the soft drink is sweet

Here πολύ means very.

• πολύ γλυκό = very sweet

Greek πολύ can function as an adverb, and in that use it does not change form:

• πολύ καλός = very good
• πολύ καλή = very good
• πολύ καλό = very good

It can also mean much / a lot, depending on context. Here, because it modifies the adjective γλυκό, the meaning is clearly very.

It is the weak genitive form.

These clitic forms are very common in Greek:

• μου = of me / to me / my
• σου = of you / to you / your
• του / της / μας / σας / τους etc.

With expressions like δίπλα, Greek often uses this genitive-style clitic:

• δίπλα μου = next to me
• δίπλα σου = next to you

You may also hear fuller versions such as:

• δίπλα σε μένα

But δίπλα μου is very natural and common.

It is functioning as the subject of είναι, so it is nominative.

Inside the clause:

• αυτό το αναψυκτικό = subject
• είναι = is
• πολύ γλυκό = predicate adjective

With neuter nouns, the nominative and accusative singular often look the same, so the form αναψυκτικό does not visibly change. Its job in the sentence tells you it is the subject here.

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

This sentence has a very neutral, straightforward order:

• Η κοπέλα δίπλα μου λέει ότι αυτό το αναψυκτικό είναι πολύ γλυκό.

But Greek can move elements around for emphasis. For example, someone might say:

• Αυτό το αναψυκτικό είναι πολύ γλυκό, λέει η κοπέλα δίπλα μου.

That would put more focus on this soft drink is very sweet.

Even so, the original version is the most neutral and easiest for learners to understand.

A rough pronunciation guide is:

ee ko-PE-la THEE-pla moo LE-i O-ti af-TO to a-na-psik-ti-KO EE-ne po-LI ghlee-KO

A few useful notes:

• η is pronounced ee
• δίπλα has the stress on the first syllable: DI-pla
• λέει is pronounced roughly LE-i
• είναι is pronounced roughly EE-ne
• γλυκό has a sound like ghlee-KO

The stress marks in Greek are important, because they show which syllable is stressed:

• κοπέλα
• δίπλα
• λέει
• αυτό
• αναψυκτικό
• πολύ
• γλυκό