Learn Hardcore Russian: Ночью я видел ужасный сон и проснулся очень рано. - At night I had a terrible dream and woke up very early.

Ночью here is the instrumental case of ночь, used adverbially to mean “at night”.
Russian often uses the instrumental (without a preposition) to express time in a general sense, like:

днём – in the daytime
утром – in the morning
вечером – in the evening
ночью – at night

You can say в ночь or в ночи, but they are more specific/poetic and usually need extra context (e.g. в ту ночь – on that night). Ночью is the most natural everyday way to say “at night” in this kind of sentence.

Why is ужасный сон in this form? Which case is сон in?

Ужасный сон is in the accusative case because it is the direct object of the verb видел.

Nominative: ужасный сон – a terrible dream (subject)
Accusative: видел ужасный сон – (I) saw a terrible dream (object)

For inanimate masculine nouns like сон, nominative and accusative look the same, so the form doesn’t change.

Why is it видел and not увидел? Aren’t both “saw”?

Both видел and увидел are past tense of “to see”, but:

видел – imperfective: focuses on the process or the fact that the action was happening or repeated.
увидел – perfective: focuses on the result or single completed moment of seeing.

With dreams, Russians usually use the imperfective to mean “I had a dream / I was dreaming”, so я видел сон = “I was seeing / I saw a dream (in general, as an experience).”

Я увидел сон sounds like a specific moment when you suddenly noticed a dream, which is unusual. So видел is the natural choice here.

How would the sentence change if the speaker is female?

For a female speaker, both past tense verbs change to feminine forms:

Я видела ужасный сон и проснулась очень рано.

The endings:

masculine: видел, проснулся
feminine: видела, проснулась (the -ся/ -сь just attaches after the verb form)

So only the verbs change; the rest of the sentence stays the same.

Why is проснулся reflexive (with -ся)? Can I just say проснул?

You must use the reflexive form проснуться in Russian to mean “to wake up”. The correct past tense masculine is проснулся.

There is no active non‑reflexive verb проснуть in normal use with this meaning.

проснуться – to wake up (reflexive, perfective)
просыпаться – to wake up, be waking up (reflexive, imperfective)

So я проснулся = “I woke up.” Я проснул is ungrammatical.

Why is проснулся perfective, but видел is imperfective in the same sentence?

The aspect choice matches how the actions are viewed:

видел (imperfective) – the dream is an ongoing experience during the night.
проснулся (perfective) – waking up is a single, completed action at a specific moment.

So the sentence structure is:

ongoing background event: I was having a terrible dream,
then a completed event: and (then) I woke up very early.

This imperfective + perfective pattern is very common in Russian storytelling.

Can I say Я видел ужасный сон ночью и проснулся очень рано? Is the word order important?

Yes, Я видел ужасный сон ночью и проснулся очень рано is grammatically correct.

Main differences:

Ночью я видел… puts ночью at the start, so the time (“at night”) is emphasized.
Я видел ужасный сон ночью… puts more neutral focus on “I saw a terrible dream”, and then adds when it happened.

Russian word order is flexible. Both versions work; the original simply highlights ночью more strongly.

Why is there no я in the second part? Could I say …и я проснулся очень рано?

You can say …и я проснулся очень рано, and it is correct.

However, in Russian, once it is clear who the subject is, the pronoun is often dropped in later coordinated clauses:

Ночью я видел ужасный сон и проснулся очень рано.

Here, it is obvious that я is still the subject of проснулся, so repeating я is not necessary. Adding я gives a bit more emphasis:

…и я проснулся очень рано can feel slightly more contrastive or emphatic: and I (as a result) woke up very early.

What is the difference between ужасный сон, страшный сон, and кошмар?

All can be related to bad dreams, but with nuance:

ужасный сон – literally “terrible/awful dream.” Focuses on how bad it was in a general sense.
страшный сон – “scary/frightening dream.” Focuses more on fear, being scared.
кошмар – “nightmare” (often as a noun on its own). You can say:
- Мне приснился кошмар. – I had a nightmare.

In your sentence, ужасный сон suggests it was really bad, possibly frightening, but the word itself is more “awful” than specifically “scary.”

What is the nuance of очень рано compared to just рано?

рано – early
очень рано – very early

Adding очень intensifies the adverb, stressing that the waking time was unusually early.

Without очень, the sentence would still be correct:

…и проснулся рано. – and (I) woke up early.

But очень makes it stronger: much earlier than normal.