Πίνω έναν μικρό εσπρέσο το πρωί, γιατί η πολλή καφεΐνη με κάνει νευρικό.

Because έναν is the masculine accusative singular form of the indefinite article ένας.

• ένας = a / one, masculine nominative
• έναν = a / one, masculine accusative
• ένα = a / one, neuter

In this sentence, εσπρέσο is treated as masculine, and it is the direct object of πίνω, so the accusative is needed: έναν εσπρέσο.

Yes. Greek grammatical gender does not always match the ending, especially with loanwords.

In everyday Greek, ο εσπρέσο is very common. Coffee names are often treated as masculine, sometimes by association with ο καφές.

Also, εσπρέσο is usually indeclinable, so the noun itself does not change form. The article and adjective show the gender and case instead:

• έναν μικρό εσπρέσο
• ο μικρός εσπρέσο would not be natural here, but it shows the idea that the article/adjective carry the grammar

Because the adjective must agree with εσπρέσο in gender, number, and case.

Here, εσπρέσο is:

• masculine
• singular
• accusative

So μικρός changes to μικρό.

Compare:

• ένας μικρός εσπρέσο = nominative
• πίνω έναν μικρό εσπρέσο = accusative

This is very common with masculine adjectives ending in -ος.

It is in the accusative case because it is the direct object of the verb πίνω.

The sentence structure is:

• Πίνω = I drink
• έναν μικρό εσπρέσο = a small espresso

Greek normally puts the direct object in the accusative.

A useful thing to notice here is that εσπρέσο itself does not visibly change, but:

• έναν shows masculine accusative
• μικρό also shows masculine accusative

It can mean either, depending on context.

In Modern Greek, the simple present often covers both:

• I drink
• I am drinking

Here, because of το πρωί and the general reason in the second clause, the meaning is clearly habitual:

• Πίνω έναν μικρό εσπρέσο το πρωί = I drink a small espresso in the morning

If the context were about what you are doing right now, πίνω could also mean I’m drinking.

Because Greek often uses the definite article in time expressions where English does not.

So:

• το πρωί = in the morning / in the mornings
• το βράδυ = in the evening / at night
• το μεσημέρι = at noon / in the afternoon

This is just the normal Greek idiom. You should learn το πρωί as a set expression.

It depends on context.

In this sentence, with the present tense and a general statement, it means something like:

• in the morning
• in the mornings
• every morning if that fits the intended meaning

It does not have to mean one specific morning. The context makes it a habitual statement.

Yes. γιατί can mean both:

• because
• why

In this sentence, it introduces a reason, so it means because:

• ..., γιατί η πολλή καφεΐνη με κάνει νευρικό.
• ..., because too much caffeine makes me nervous.

As a question word, it means why:

• Γιατί πίνεις εσπρέσο;
• Why do you drink espresso?

The meaning comes from the sentence structure and intonation.

Because καφεΐνη is a feminine noun, and the adjective must agree with it.

So:

• πολλή καφεΐνη = much / a lot of caffeine
• πολύ καφεΐνη would be wrong here

Compare:

• πολλή ζάχαρη = a lot of sugar
• πολλός καφές = a lot of coffee
• πολύ νερό = a lot of water

The forms are:

• πολλός masculine
• πολλή feminine
• πολύ neuter

Greek uses the definite article more often than English does.

Here, η πολλή καφεΐνη is the subject of the clause:

• η πολλή καφεΐνη με κάνει νευρικό

It has the sense of:

• too much caffeine
• a lot of caffeine
• more literally, the much caffeine

In English we would not normally say the much caffeine, but in Greek the article is natural.

Without the article, πολλή καφεΐνη is also possible in some contexts, but η πολλή καφεΐνη sounds more like a complete noun phrase functioning as the subject.

με is the unstressed accusative pronoun meaning me.

So:

• με κάνει = makes me

Greek usually uses these short object pronouns before the verb:

• με = me
• σε = you
• τον / τη / το = him / her / it

εμένα is the stressed form, used for emphasis or contrast:

• Εμένα με κάνει νευρικό. = It makes me nervous.

In a neutral sentence, με is the normal choice.

Here νευρικό is correct because it agrees with the person referred to by με, and it functions as a complement after κάνει.

The pattern is:

• κάτι/κάποιος με κάνει + adjective
• something/someone makes me + adjective

For a male speaker, you say:

• με κάνει νευρικό

For a female speaker, you would say:

• με κάνει νευρική

So the adjective changes according to the person being described, not according to καφεΐνη.

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

The sentence as given is natural:

• Πίνω έναν μικρό εσπρέσο το πρωί, γιατί η πολλή καφεΐνη με κάνει νευρικό.

But you could also say:

• Το πρωί πίνω έναν μικρό εσπρέσο...

That puts more emphasis on the morning.

Similarly, Greek often moves elements around for emphasis, topic, or style, while keeping the same basic meaning.