Learn Hardcore Russian: В тот день мы должны были спрятаться под крышей кафе, потому что началась сильная гроза. - That day we had to hide under the café roof because a strong thunderstorm began.

в + Accusative (в тот день) is the normal way to say “on [a particular day]” in Russian.
тот = that (referring to a day already known from context or a story).
этот = this (closer, more immediate; often “this day / today” or “this day we’re in now”).

So:

В тот день… – On that day… (some past day that’s already identified in the narrative).
В этот день… – On this day… (this specific day, maybe “today” or in a more present-focused story).

На тот день is used for a different meaning: “for that day” in the sense of scheduling or plans:

Мы назначили встречу на тот день. – We scheduled the meeting for that day.

Here we’re not talking about scheduling; we’re describing what happened on a specific past day, so в тот день is correct.

Why do we say мы должны были спрятаться for past obligation and not just мы должны спрятаться?

Russian marks past tense for должен using быть in the past:

мы должны спрятаться – we have to hide / we must hide (present/future obligation).
мы должны были спрятаться – we had to hide (obligation in the past).

Structure:

должны – short-form adjective (plural), “obliged, supposed to”.
были – past plural of быть (to be).
Together: должны были + infinitive = “had to / were supposed to + verb”.

So to talk about an obligation that existed in the past, you almost always need были (or another past form: должен был, должна была, должно было).

What is the difference between спрятаться and прятаться here?

They differ in aspect and meaning:

спрятаться – perfective, reflexive; means “to hide oneself (successfully), to get hidden” as a completed action.
прятаться – imperfective, reflexive; means “to be hiding, to hide (in general, repeatedly, for some time)”.

In context:

мы должны были спрятаться = we had to hide (take cover) – one-time, goal-oriented action: go from “not sheltered” to “sheltered”.
мы должны были прятаться would sound like we had to keep hiding (for some period of time) – focuses on the ongoing process, not the moment of taking cover.

Because the sentence describes the single act of taking shelter when the storm started, спрятаться (perfective) is more natural.

Why does спрятаться end in -ся? What does that mean?

The -ся ending marks a reflexive or “middle” verb form.

Base verb: спрятать – to hide (something), transitive.
- спрятать книгу – to hide a book.
Reflexive form: спрятаться – literally to hide oneself, i.e. to get (oneself) hidden → to hide, to take cover.

So:

спрятать кого/что? – to hide someone/something.
спрятаться (самому) – to hide yourself / to go into hiding.

In this sentence мы должны были спрятаться means “we had to hide (ourselves)”, which is why the reflexive -ся is used.

Why is it под крышей and not под крышу?

The preposition под can take either Instrumental or Accusative, with different meanings:

под + Instrumental → location, “under, beneath (and staying there)”
- под крышей – under the roof (static location).
под + Accusative → direction, “to under, go under (movement to that place)”
- под крышу – (go) under the roof (movement into that position).

In the sentence:

спрятаться под крышей кафе focuses on ending up under the roof, being located there for shelter.
If you said спрятаться под крышу, you would emphasize the movement to that position (“to go under the roof”), but it’s less usual here; with спрятаться we normally talk about the resulting place, so под крышей is standard.

Why is крышей in the Instrumental case after под?

Because with the meaning of location (“under X and staying there”), под requires the Instrumental case.

Nominative: крыша – roof
Instrumental: крышей – with/by/under the roof

So:

под крышей – under the roof (where you are).
под крышу – to under the roof (where you are going – Accusative, direction).

In this sentence, they hide and end up under the roof, so it’s a static location → Instrumental: под крышей.

Why doesn’t кафе change form in под крышей кафе?

Кафе is one of those Russian nouns that are indeclinable:

It keeps the same form in all cases: Nom., Gen., Dat., Acc., Instr., Prep.

So:

кафе (Nom.) – the café
в кафе (Prep.) – in the café
крыша кафе (Gen.) – the roof of the café
под крышей кафе – under the roof of the café

In под крышей кафе:

крышей – Instrumental (after под, as explained).
кафе – syntactically is in the Genitive (it tells whose roof), but its form doesn’t change because the word is indeclinable.

Could we change the word order, like В тот день мы спрятались под крышей кафе, потому что должна была начаться сильная гроза? How flexible is the order here?

Russian word order is more flexible than English, but you must keep agreement and tense correct:

Changing positions:
- В тот день мы должны были спрятаться под крышей кафе…
- Мы в тот день должны были спрятаться под крышей кафе…
  Both are fine; the nuance is where you put the emphasis (on that day or on we).
The second part:
- Natural variants:
  - …потому что началась сильная гроза.
  - …потому что сильная гроза началась. Word order changes emphasis, but both are correct.

Your suggested change должна была начаться сильная гроза means “a strong thunderstorm was supposed to start”, not “started”. That changes the meaning:

началась сильная гроза – a strong thunderstorm started (it actually began).
должна была начаться сильная гроза – a strong thunderstorm was supposed to start (plan/expectation, maybe it did, maybe not).

So you can move parts around, but don’t change the forms if you want to keep the same meaning.

Why do we need a comma before потому что?

Потому что introduces a subordinate clause of reason (a “because”-clause), so Russian punctuation rules require a comma before it in most cases.

Structure:

Main clause: В тот день мы должны были спрятаться под крышей кафе,
Subordinate clause (reason): потому что началась сильная гроза.

Thus:

…, потому что … = “, because …” → comma is standard.

There are rare, more fixed-expression cases where потому что may be written without a comma (e.g., in some shortened colloquial phrases), but here it’s a normal full subordinate clause, so the comma is obligatory.

What is the difference between потому что and так как in this sentence?

Both can mean “because”, but they differ in style and emphasis:

потому что – neutral, very common; works in almost any context.
так как – slightly more formal / bookish; often used when the reason is already known or is less important than the main statement.

In your sentence:

…потому что началась сильная гроза. – neutral: because a strong thunderstorm started.
…так как началась сильная гроза. – sounds a bit more formal or explanatory, like you’re just giving background.

For everyday spoken Russian, потому что is the most natural here.

Why is it началась сильная гроза and not начался сильный гроза or something else?

Russian verbs in the past tense agree in gender and number with the subject.

Subject: гроза – feminine noun (Nom. sg.).
Adjective: сильная – feminine nominative singular.
Verb: past tense must also be feminine singular.

So:

Masculine: начался (e.g., начался дождь – the rain started).
Feminine: началась (e.g., началась гроза – the storm started).
Neuter: началось.
Plural: начались.

That’s why it must be началась сильная гроза: all three agree in feminine singular.

“начался сильный гроза” is ungrammatical because гроза is feminine, but начался / сильный are masculine.

Why is the verb началась perfective? Could we say начиналась сильная гроза?

Начаться vs начинаться is again about aspect:

начаться (perf.) – to begin, to start (as a completed event, the moment of beginning).
- началась сильная гроза – a strong thunderstorm started (it actually began).
начинаться (impf.) – to be starting, to begin (in progress or repeated).
- начиналась сильная гроза – a strong thunderstorm was starting / would start (repeatedly).

In your sentence, the storm started at a clear point in time, causing them to hide. That’s a single completed event → perfective началась fits best.

Начиналась сильная гроза could work in a more descriptive, ongoing background sense, for example:

Вдалеке начиналась сильная гроза, и небо постепенно темнело.
In the distance a strong thunderstorm was starting, and the sky was gradually darkening.

But if the storm is the concrete reason they took cover, the perfective началась is more natural.

What is the difference in meaning between мы должны были спрятаться and нам пришлось спрятаться?

Both can translate as “we had to hide”, but they have different shades of meaning:

мы должны были спрятаться
- Literally: we were obliged / supposed to hide.
- Focus: obligation, necessity, plan, rule, external requirement.
- Neutral about whether it was pleasant or not.
нам пришлось спрятаться
- Literally: it turned out that (for us) it was necessary to hide → we were forced / ended up having to hide.
- Focus: circumstances forced us, it was unavoidable, often with a feeling of inconvenience or lack of choice.

In this context, both could be used:

В тот день нам пришлось спрятаться под крышей кафе, потому что началась сильная гроза.
Emphasizes that the sudden storm forced them to take shelter.
В тот день мы должны были спрятаться под крышей кафе…
Slightly more neutral: there was a necessity/obligation to hide because of the storm.

So the sentence as written is fine; using нам пришлось would just shift the nuance toward “we were compelled by circumstances.”

Is there any special reason for saying под крышей кафе rather than something like в кафе?

Yes, the preposition choice changes the imagery:

в кафе – in the café (inside the building).
под крышей кафе – literally under the café’s roof.

Под крышей кафе suggests:

They might be under a canopy, an awning, or on a covered terrace/veranda.
It emphasizes being protected by the roof from the rain, not necessarily going fully inside the building.

So под крышей кафе paints a more concrete picture of taking shelter under the physical roof (for example, under the awning outside), which fits well with the idea of suddenly hiding from a thunderstorm.