Por la noche, el hombre dice buenas noches.

In Spain, por la noche is the most natural general expression for at night / in the evening.

• Por la noche = a general time of day
• En la noche = much less common in Spain in this kind of sentence, and may sound more literary, regional, or tied to a specific situation

You may also hear de noche, especially in contrasts like Trabajo de noche y duermo de día.

Spanish often uses the definite article with parts of the day:

• por la mañana
• por la tarde
• por la noche

So this is just the normal fixed pattern. English often says in the morning or at night, but Spanish keeps the article here.

Spanish usually needs an article before a singular countable noun when you mean the man or a man in a full sentence.

• el hombre = the man
• un hombre = a man

Using just hombre here would sound incomplete in standard Spanish.

Dice is the third-person singular form of decir in the present tense.

Present tense of decir:

• yo digo
• tú dices
• él / ella / usted dice
• nosotros decimos
• vosotros decís
• ellos / ellas / ustedes dicen

So el hombre dice means the subject is he / the man.

Because decir and hablar are not the same.

• decir = to say
• hablar = to speak / to talk

Here the sentence gives the actual words the man says, so decir is the right verb.
If you used habla, it would mean more that he speaks or is talking, not specifically that he says those words.

This greeting is simply fixed in the plural in Spanish, just like:

• buenos días
• buenas tardes
• buenas noches

You do not normally say buena noche as the standard greeting. The plural form is the conventional one.

It can mean either, depending on context.

In Spain, buenas noches is used:

• when greeting someone at night
• when leaving at night
• sometimes when going to bed

So the sentence only tells you the man says buenas noches; context would tell you whether he is arriving, leaving, or ending the day.

In a simple teaching sentence, Spanish often leaves out quotation marks even when reporting the exact words. That is common in textbooks and examples.

In more careful writing, you could also write:

• Por la noche, el hombre dice: buenas noches.
• Por la noche, el hombre dice «buenas noches».

So the version without quotation marks is not strange in a learning context.

Because Por la noche is placed at the beginning as a time expression. Spanish often separates that kind of introductory phrase with a comma.

• Por la noche, el hombre dice buenas noches.

The comma helps readability. In short simple sentences, punctuation can vary a little, but this version is very natural.

Yes, but some versions sound more natural than others.

Most natural here:

• Por la noche, el hombre dice buenas noches.

Also possible:

• El hombre dice buenas noches por la noche.

That second version is grammatical, but it sounds a bit heavier because por la noche comes late and repeats the idea of noches. The original order is smoother.

Because greetings are normally not capitalized in Spanish unless they begin the sentence.

So:

• El hombre dice buenas noches.
• Buenas noches.

Only the second one starts with a capital because it begins the sentence.

It is grammatical, but it is a little repetitive because you have:

• por la noche
• buenas noches

A textbook may use it on purpose to reinforce vocabulary. In real conversation, a native speaker might choose a less repetitive sentence if style mattered. But as a learner example, it is completely fine.