Ο οδηγός είναι αθώος.

A rough pronunciation is:

• Ο → o
• οδηγός → o-thi-GHOS
• είναι → EE-ne
• αθώος → a-THO-os

A few useful notes:

• δ here sounds like th in this
• γ before ο is a soft throat sound, not a hard English g
• αθώος has three syllables: α-θώ-ος

So the whole sentence is roughly:

o o-thi-GHOS EE-ne a-THO-os

Ο is the masculine singular definite article, so here it means the.

So:

• ο οδηγός = the driver

It also tells you that οδηγός is being treated as masculine in this sentence.

είναι is the present-tense form of είμαι, which means to be.

Here it means is:

• ο οδηγός είναι αθώος = the driver is innocent

A useful extra fact: είναι is also the form for they are, so Greek learners often notice that this verb is a bit irregular.

Because αθώος is being used as a predicate adjective after είναι.

So this pattern means:

• ο οδηγός είναι αθώος = the driver is innocent

If you put the adjective directly with the noun, you get a different structure:

• ο αθώος οδηγός = the innocent driver

So:

• ο οδηγός είναι αθώος = statement about the driver
• ο αθώος οδηγός = noun phrase describing the driver

Because it agrees with ο οδηγός.

In Greek, adjectives must agree with the noun they describe in:

• gender
• number
• often case

Here:

• ο οδηγός is masculine singular
• so αθώος is also masculine singular

That is why you do not see a different ending such as a feminine one.

Yes. Οδηγός can be used for a male or female driver. What changes is usually the article and any adjective that agrees with it.

For a male driver:

• Ο οδηγός είναι αθώος.

For a female driver:

• Η οδηγός είναι αθώα.

Notice:

• the noun οδηγός stays the same
• the article changes from ο to η
• the adjective changes from αθώος to αθώα

Because αθώος is not being used attributively inside a noun phrase. It is part of the predicate after είναι.

Compare:

• ο αθώος οδηγός = the innocent driver
• ο οδηγός είναι αθώος = the driver is innocent

In the first sentence, the adjective is inside the noun phrase, so articles matter differently.
In the second, it is a predicate adjective, so you simply use είναι + adjective.

The order Ο οδηγός είναι αθώος is the most neutral and straightforward.

But Greek word order is fairly flexible, so other orders are possible for emphasis, for example:

• Αθώος είναι ο οδηγός.
  This can emphasize innocent

• Ο οδηγός αθώος είναι.
  Possible in special contexts, but less neutral

So the original sentence is the best basic version for learners.

Οδηγός is in the nominative singular, because it is the subject of the sentence.

You can tell from:

• the article ο
• the basic sentence role: the driver is the one being talked about

Αθώος is also nominative singular because predicate adjectives after είναι normally agree with the subject.

So both match:

• ο οδηγός → masculine nominative singular
• αθώος → masculine nominative singular

Yes. Accent marks show which syllable is stressed, and stress is important in Greek pronunciation.

In this sentence:

• οδηγός → stress on the last syllable
• είναι → stress on the first syllable
• αθώος → stress on θώ

These marks are not optional in normal writing. They help you pronounce the words correctly and can sometimes distinguish forms.

You add δεν before the verb:

• Ο οδηγός δεν είναι αθώος.

So:

• είναι = is
• δεν είναι = is not

This is the normal way to negate a sentence with είμαι.