Мы говорим «я так и не...», когда что‑то ожидалось, но так и не случилось.

Yes. Так и не is best learned as a fixed expression, or at least a very stable pattern.

It does not mean simply so and not word for word. In this construction, it adds the idea that:

• something was expected,
• or there was a chance for it to happen,
• but in the end it still did not happen.

So я не прочитал книгу is just I didn’t read the book, but я так и не прочитал книгу suggests something more like:

• I never did read the book
• I still didn’t end up reading the book

The extra feeling is often: expectation, delay, and final non-fulfillment.

Not really. In this expression, и is not functioning like the normal English and.

In так и не, the и is a particle that helps create the idiomatic meaning of still never / never did / failed to after all. It strengthens the sense that something remained unrealized despite expectation.

So you should usually treat так и не as a unit, not try to translate each word separately.

The usual pattern is:

subject + так и не + verb

For example:

• Я так и не понял. — I never did understand.
• Он так и не пришёл. — He never came after all.
• Мы так и не встретились. — We never ended up meeting.

So the negative particle не goes directly before the verb, just as usual, but here it is preceded by так и, forming the set phrase так и не.

You can use it with any subject.

Examples:

• Я так и не позвонил. — I never did call.
• Ты так и не ответил. — You never answered after all.
• Он так и не приехал. — He never arrived.
• Мы так и не нашли время. — We never found the time.
• Они так и не договорились. — They never reached an agreement.

So я так и не... is just one common model, but the structure is fully productive with other persons too.

It can be used in different tenses, although the past is very common.

Examples:

• Я так и не понял. — I never did understand.
• Я так и не понимаю. — I still don’t understand.
• Я так и не пойму. — I apparently still won’t understand / I’m not going to understand after all.

The past tense is especially common because this pattern often describes an expected result that failed to happen by some later point.

This is one of the most important questions.

не + verb is a plain negation:

• Я не позвонил. — I didn’t call.

так и не + verb adds the idea that calling was expected, intended, possible, or waited for, but it never happened:

• Я так и не позвонил. — I never did call.

So так и не often sounds like:

• still didn’t
• never did
• failed to
• didn’t end up

It often carries a shade of disappointment, incompleteness, or contrast with expectation.

Very often, perfective verbs follow так и не when we are talking about a result that never happened:

• так и не пришёл — never came
• так и не сделал — never did
• так и не понял — never understood

This is because the speaker is focusing on the expected completion or result, and saying that result never occurred.

But imperfective is also possible, especially when the meaning is about an ongoing state that still has not changed:

• Я так и не знаю, что произошло. — I still don’t know what happened.
• Он так и не спит по ночам. — He still doesn’t sleep at night.

So a useful rule is:

• use perfective when an expected event/result never happened;
• use imperfective when an expected state or repeated action still has not become reality.

Мы говорим literally means we say, and in Russian it is often used to describe a general rule of usage, just like English does.

So here it means:

• We say ... when ...
• in other words, This expression is used when ...

It is not talking about a specific group of people speaking right now. It is a general statement about how Russian works.

Both are past-tense forms, but they are different in aspect and in how they present the situation.

• ожидалось is imperfective past: was expected
• случилось is perfective past: happened

That fits the meaning very well:

• first, there was an expectation in progress or in the background: что‑то ожидалось
• then, the result did not occur: но так и не случилось

So the sentence describes a background expectation followed by a failed result.

Because the subject is что‑то, and что‑то is grammatically neuter singular in Russian.

So the past tense of the verb must agree with it:

• masculine: случился
• feminine: случилась
• neuter: случилось

That is why we get:

• что‑то случилось — something happened
• что‑то не случилось — something did not happen
• что‑то так и не случилось — something never ended up happening

It is completely normal and very common standard Russian.

You can hear it in:

• everyday speech,
• books,
• journalism,
• formal and informal writing.

It is not slang. It is simply a very useful and natural way to express that something expected never happened.

Yes, very often.

In that case, так и не usually goes before the verb that is being negated, or before the infinitive phrase as a whole depending on the structure.

Common examples:

• Я так и не смог понять. — I never managed to understand.
• Мы так и не успели поговорить. — We never managed to talk.
• Он так и не хотел отвечать. — He still did not want to answer.

A very frequent pattern is:

• так и не смог + infinitive
• так и не успел + infinitive

These are extremely natural in Russian.

Not always strongly emotional, but it usually suggests that the speaker sees the non-event as meaningful in some way.

It often implies one of these:

• it was expected,
• it was planned,
• it was possible,
• someone was waiting for it,
• and yet it never happened.

Sometimes the tone is disappointed:

• Я так и не встретил его. — I never did meet him.

Sometimes it is just factual:

• Письмо так и не пришло. — The letter never arrived.

So the expression carries a nuance, but the strength of the emotion depends on context.