Я вздохнул и ничего не сказал.

Because it is a past tense verb.

In Russian, past tense is formed from the infinitive stem plus a form that agrees with gender and number:

• вздохнул = masculine singular
• вздохнула = feminine singular
• вздохнуло = neuter singular
• вздохнули = plural

So Я вздохнул means I sighed if the speaker is male. A female speaker would say Я вздохнула.

For the same reason: it is another past tense verb agreeing with the speaker.

Since the subject is я, Russian past tense still shows gender:

• Я сказал = I said (male speaker)
• Я сказала = I said (female speaker)

So the sentence as written sounds like it is spoken by a man.

Yes, Russian can often omit subject pronouns, especially when the meaning is clear from context.

So both of these are possible:

• Я вздохнул и ничего не сказал.
• Вздохнул и ничего не сказал.

Including я can make the subject more explicit, more contrastive, or simply more natural in that context. It is not always required, but it is perfectly normal.

Вздохнул means sighed or let out a sigh.

It is the perfective past form of вздохнуть, which focuses on a single completed action: one sigh.

Its imperfective partner is usually вздыхать:

• Он вздохнул. = He sighed. / He gave a sigh.
• Он вздыхал. = He was sighing. / He sighed repeatedly.

Here, вздохнул fits because the sentence describes one completed action followed by another: sighed and said nothing.

Because after a negative verb, Russian commonly uses ничего to mean nothing.

In this sentence:

• не сказал = did not say
• ничего = nothing

Together: ничего не сказал = said nothing

A few notes:

• ничего is the usual form in everyday Russian after negation.
• ничто exists, but it is much more literary or formal.
• нечто means something, not nothing.

So ничего не сказал is the normal, idiomatic way to say didn’t say anything / said nothing.

Yes, and that is normal Russian grammar.

Russian regularly uses negative concord, which means multiple negative elements appear together:

• Я ничего не знаю. = I know nothing. / I don’t know anything.
• Он никого не видел. = He saw nobody. / He didn’t see anyone.

So:

• ничего = nothing
• не сказал = did not say

Together they make the standard Russian structure for said nothing. This is not considered incorrect or illogical in Russian.

Because it is the genitive form of ничто used with negation.

The basic pattern is:

• что → ничего in this kind of negative construction

After many negated transitive verbs, Russian uses the genitive rather than the accusative:

• Я не видел брата.
• Он не пил воды.
• Она ничего не сказала.

So ничего is the expected form after не сказал.

Because сказать and говорить are not exactly the same.

• сказать is usually perfective and often means say, tell, utter
• говорить is imperfective and often means speak, talk, say in a more ongoing or repeated sense

Here the idea is that the speaker did not say a thing, meaning no single utterance was made. That matches сказать very naturally:

• ничего не сказал = said nothing

If you used не говорил, it could suggest was not speaking, didn’t talk, or focus more on the process rather than the absence of any particular remark.

И means and.

It simply joins the two actions:

• Я вздохнул = I sighed
• и ничего не сказал = and said nothing

Russian often uses и just like English and to connect verbs with the same subject.

The given word order is very natural:

• Я вздохнул и ничего не сказал.

But Russian word order is flexible, and changes can affect emphasis:

• Я ничего не сказал и вздохнул. = puts said nothing first
• Ничего я не сказал. = emphasizes I said nothing
• Я вздохнул и не сказал ничего. = possible, but a bit less neutral than ничего не сказал

The most neutral and common phrasing here is exactly the one you were given.

It can mean both.

In English, these are usually equivalent:

• I said nothing
• I didn’t say anything

Russian uses one structure:

• Я ничего не сказал.

So depending on context and translation style, either English version may be used.

Yes. It most naturally describes a short sequence:

1. I sighed
2. and said nothing

Because both verbs are in the past, and both are perfective, the sentence presents two completed actions or events in narrative order.

It gives a very common storytelling effect in Russian: a brief action followed by silence.

Yes. A female speaker would change the past tense verb endings:

• Я вздохнула и ничего не сказала.

Only the past tense verb forms change to match feminine gender. The rest of the sentence stays the same.