Σε εκείνη τη στροφή υπάρχει πάντα μια μεγάλη πινακίδα, γιατί πολλά αυτοκίνητα στρίβουν γρήγορα.

Because σε + accusative is the normal way to say location or direction in Modern Greek.

Here:

• σε = at, on, in, to
• εκείνη = that
• τη στροφή = the turn / the bend

So Σε εκείνη τη στροφή means at that bend/turn.

In English, we often begin with the location too: At that bend, there is always... Greek does the same very naturally.

Because εκείνη must agree with στροφή.

The noun στροφή is:

• feminine
• singular

So the demonstrative also has to be:

• feminine
• singular

That gives:

• masculine: εκείνος
• feminine: εκείνη
• neuter: εκείνο

Since στροφή is feminine, Greek uses εκείνη.

Both exist, but τη στροφή is very normal in Modern Greek.

The feminine accusative article is basically την, but the final -ν is often dropped before many consonants in everyday standard Greek. Since στροφή begins with σ, you commonly get:

• τη στροφή

You may also see or hear την στροφή, especially in more careful or formal speech/writing, but τη στροφή is completely standard and natural.

Because the thing that exists is singular: μια μεγάλη πινακίδα.

υπάρχει means there is / exists
υπάρχουν means there are / exist

So:

• υπάρχει μια μεγάλη πινακίδα = there is a big sign
• υπάρχουν μεγάλες πινακίδες = there are big signs

Even though English uses there is, Greek just uses the verb υπάρχει directly.

No. It is better understood as the thing that exists, so functionally it is the subject.

Greek often puts that noun phrase after the verb:

• Υπάρχει μια μεγάλη πινακίδα

English uses a dummy subject:

• There is a big sign

Greek does not need an equivalent of English there in this structure.

Because μια is the feminine singular form of the indefinite article, meaning a / one.

Since πινακίδα is feminine singular, Greek uses:

• μια πινακίδα = a sign

Compare:

• ένας for masculine nouns
• μια / μία for feminine nouns
• ένα for neuter nouns

So μια μεγάλη πινακίδα means a big sign.

Because adjectives in Greek must agree with the noun they describe in:

• gender
• number
• case

The noun πινακίδα is feminine singular, so the adjective must also be feminine singular:

• μεγάλος = masculine
• μεγάλη = feminine
• μεγάλο = neuter

So:

• μια μεγάλη πινακίδα = a big sign

This agreement is one of the most important features of Greek grammar.

πάντα means always.

It modifies the whole situation:

• υπάρχει πάντα μια μεγάλη πινακίδα = there is always a big sign

Its placement is fairly flexible in Greek. You could also hear:

• Πάντα υπάρχει μια μεγάλη πινακίδα
• Υπάρχει μια μεγάλη πινακίδα πάντα — less neutral in this sentence, but possible in some contexts

The version in your sentence sounds natural and neutral.

Because αυτοκίνητα is a neuter plural noun.

The singular is:

• το αυτοκίνητο = the car

The plural is:

• τα αυτοκίνητα = the cars

So πολλά must also be neuter plural:

• πολλά αυτοκίνητα = many cars

Compare:

• πολλοί άνθρωποι = many people, masculine plural
• πολλές γυναίκες = many women, feminine plural
• πολλά αυτοκίνητα = many cars, neuter plural

στρίβουν is the 3rd person plural present tense of στρίβω, meaning to turn.

So:

• στρίβω = I turn
• στρίβεις = you turn
• στρίβει = he/she/it turns
• στρίβουν = they turn

Since the subject is πολλά αυτοκίνητα = many cars, Greek uses the plural verb:

• πολλά αυτοκίνητα στρίβουν γρήγορα = many cars turn quickly

The present tense here expresses a habitual or general fact, not something happening only once.

Yes. They come from the same basic idea of turning.

• στροφή = a turn, bend, curve
• στρίβουν = they turn

That is very useful for vocabulary building. Greek often has families of related words like this, so once you know one root, other words become easier to recognize.

Because here the word modifies the verb, not the noun.

• γρήγορα = quickly
• γρήγορα αυτοκίνητα = fast cars

In the sentence:

• πολλά αυτοκίνητα στρίβουν γρήγορα

the meaning is that the cars turn quickly, so Greek uses the adverb γρήγορα.

This is exactly like English:

• quick cars = adjective
• turn quickly = adverb

Yes. γιατί can mean either because or why, depending on context.

Here it means because, since it introduces a reason:

• υπάρχει πάντα μια μεγάλη πινακίδα, γιατί...
• there is always a big sign, because...

In a question, it can mean why:

• Γιατί στρίβουν γρήγορα;
• Why do they turn quickly?

So the meaning is determined by the sentence structure and punctuation.

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

Your sentence:

• Σε εκείνη τη στροφή υπάρχει πάντα μια μεγάλη πινακίδα, γιατί πολλά αυτοκίνητα στρίβουν γρήγορα.

Other possible versions include:

• Πάντα υπάρχει μια μεγάλη πινακίδα σε εκείνη τη στροφή...
• Μια μεγάλη πινακίδα υπάρχει πάντα σε εκείνη τη στροφή...

These versions are grammatically possible, but they shift emphasis.

The original sentence is natural because it:

1. sets the location first,
2. gives the main fact,
3. then gives the reason.

That is a very common and clear Greek sentence pattern.