Ο τόνος είναι στη δεύτερη συλλαβή.

A natural pronunciation is roughly:

o TO-nos EE-ne sti THEF-te-ri si-la-VEE

A more careful breakdown:

• Ο = o
• τόνος = TO-nos
• είναι = EE-ne
• στη = sti
• δεύτερη = THEF-te-ri or DEV-te-ri, depending on speech and accent
• συλλαβή = si-la-VEE

The stressed syllables are the ones marked by the written accent:

• τόνος
• είναι
• δεύτερη
• συλλαβή

στη is the very common contracted form of:

• σε = in / at / on / to
• τη(ν) = the (feminine singular)

So:

• σε τη συλλαβή → στη συλλαβή

In modern Greek, this contraction is standard and sounds much more natural in everyday language.

It is in the accusative singular.

That is because the preposition σε normally takes the accusative in modern Greek. Since στη = σε + τη(ν), the noun after it is accusative:

• nominative: η συλλαβή
• accusative: τη συλλαβή

The adjective changes with it:

• nominative: η δεύτερη συλλαβή
• accusative: τη δεύτερη συλλαβή

In this sentence, after στη, you have the accusative form.

Because δεύτερη has to agree with συλλαβή.

συλλαβή is:

• feminine
• singular

So the adjective must also be:

• feminine
• singular

That gives:

• masculine: δεύτερος
• feminine: δεύτερη
• neuter: δεύτερο

Since the noun is συλλαβή, the correct form is δεύτερη.

Because ο here is the masculine singular definite article, and Greek articles like ο, η, το are normally not accented.

Greek usually puts a written accent on words with more than one syllable, but many very short function words, especially one-syllable articles, do not carry one.

So:

• ο τόνος = the stress / accent
• η συλλαβή = the syllable
• το γράμμα = the letter

In this sentence, τόνος means stress or accent.

In Greek, τόνος can refer to:

• the stress of a word
• the written accent mark
• more generally, a tone

In a sentence like this one, it usually means the word stress: the stressed syllable is the second one.

So Ο τόνος είναι στη δεύτερη συλλαβή means the stress falls on the second syllable.

Because the written accent mark in Greek shows which syllable is stressed in that word.

So in τόνος, the first syllable is stressed:

• ΤΟ-nos

The fact that the word means stress/accent does not change the normal spelling rules. It still follows ordinary Greek accentuation and therefore gets its own written accent mark.

Yes, very close to that.

Greek often uses σε for ideas that English might express with:

• in
• on
• at

So στη δεύτερη συλλαβή is literally something like:

• in/on the second syllable

In English, we usually say:

• The stress is on the second syllable.

Greek uses its own normal preposition pattern here.

That is just part of the word’s grammatical gender.

In Greek, every noun has a grammatical gender:

• masculine
• feminine
• neuter

συλλαβή happens to be a feminine noun, which is why it goes with:

• η in the nominative
• τη(ν) in the accusative
• feminine adjective forms like δεύτερη

This gender is grammatical, not logical. It is something you learn together with the noun.

Yes, absolutely.

That is another very common way to say it. It literally means:

• The stress falls on the second syllable.

Both are natural:

• Ο τόνος είναι στη δεύτερη συλλαβή.
• Ο τόνος πέφτει στη δεύτερη συλλαβή.

The second version may sound a bit more idiomatic when talking specifically about word stress, but the sentence you were given is perfectly understandable and correct.

The main dictionary forms are:

• ο = the masculine singular article
• τόνος = stress, accent, tone
• είμαι = to be
• σε = in, at, on, to
• δεύτερος / δεύτερη / δεύτερο = second
• συλλαβή = syllable

In the sentence:

• είναι is the he/she/it is form of είμαι
• στη is the contracted form of σε τη
• δεύτερη is the feminine singular form agreeing with συλλαβή