Learn Hardcore Russian: Перед сном я делаю лёгкую растяжку спины и плеч. - Before sleep I do a light stretch for my back and shoulders.

The instrumental case is used after certain prepositions, including перед.
In this sentence, the instrumental doesn’t add extra meaning by itself; it’s simply required by перед: перед сном = before sleep / before going to bed.

Could I say до сна instead of перед сном? What’s the difference?

You can say до сна, but it sounds less natural in everyday speech for this specific idea.

перед сном – very common, idiomatic: before bed, before going to sleep. Suggests the time right before you fall asleep, part of a nightly routine.
до сна – literally until sleep / before sleep happens. Grammatically correct, but used less often in this everyday context; it can sound a bit more formal or neutral.

For a nightly routine, Russians strongly prefer перед сном.

Why is it лёгкую and not лёгкий?

Because adjectives must agree with the noun they describe in gender, number, and case.

Noun: растяжку
- Dictionary form: растяжка – feminine
- In the sentence: растяжку – feminine, singular, accusative (direct object)

So the adjective лёгкий (light) has to match:

Masculine: лёгкий
Feminine: лёгкая (nom.), лёгкую (acc.)
Neuter: лёгкое

Since растяжку is feminine accusative singular, the adjective must be лёгкую:

лёгкую растяжку – a light / gentle stretch.

What exactly does лёгкая / лёгкую mean here: “light” or “easy”?

Literally, лёгкий means light (not heavy) or easy. In the context of exercise:

лёгкая растяжка = a light, gentle, not intense stretch.
It implies low effort, low intensity, “just a bit of stretching”.

So here лёгкую растяжку is best understood as a gentle stretch.

Why is it растяжку and not растяжка?

Растяжку is the accusative case, used for the direct object of the verb.

Dictionary form: растяжка (nominative, feminine)
Accusative (what do I do?): делаю что? → лёгкую растяжку

Pattern:

Я делаю что? – растяжку.
I do a stretch.

So растяжка → растяжку because it receives the action of делаю (“I do / I am doing”).

Why does the sentence use делаю растяжку instead of a verb like “to stretch” (растягиваться)?

Russian often uses a “do + noun” structure where English prefers a single verb.

делать растяжку – literally “to do a stretch / to do stretching”
English equivalent: to stretch / to do some stretching

You can use a verb:

я растягиваюсь перед сном – I stretch before bed.

But:

делать растяжку is very common, especially in fitness / routine contexts.
It sounds natural and neutral, like “do some stretching” in English.

So Я делаю лёгкую растяжку… = I do a light stretch… / I do some gentle stretching…

Why are спины and плеч in those forms? What cases are they?

They’re in the genitive case, showing what the stretch is for / of.

спина (back)
- Nominative: спина
- Genitive singular: спины
плечо (shoulder)
- Nominative plural: плечи
- Genitive plural: плеч

After растяжка (stretch) you often use the genitive to say stretch of what?:

растяжка чего? – спины и плеч
a stretch of the back and shoulders

So:

лёгкую растяжку (чего?) спины и плеч
a light stretch of the back and shoulders.

Why is it плеч, not плечей?

Both forms exist historically, but for the noun плечо the standard genitive plural is плеч.

Declension of плечо (shoulder):

Nom. sg.: плечо
Nom. pl.: плечи
Gen. pl.: плеч

You’ll see плеч in correct, modern standard Russian:

широкие плечи → ширина плеч – broad shoulders → width of the shoulders
растяжка плеч – stretching of the shoulders

So плеч here is the normal, correct genitive plural form.

Could I say спину и плечи instead of спины и плеч?

Not with растяжку in this structure.

There are two different constructions:

With a noun of action + genitive:
- лёгкую растяжку (чего?) спины и плеч
  a light stretching of the back and shoulders
Here, спины and плеч must be genitive, because they depend on растяжку (stretch of what?).
With a verb + direct object:
- я растягиваю спину и плечи – I stretch my back and shoulders.
Here, спину and плечи are accusative as direct objects of the verb растягиваю.

So:

With делаю растяжку → use genitive: спины и плеч
With растягиваю → use accusative: спину и плечи

What does делаю mean here in terms of time/aspect? Why not use a future or perfective form like сделаю?

Делаю is present tense, imperfective aspect:

я делаю – I do / I am doing (repeated, habitual, or in progress)

In Russian, present imperfective is used for:

habits / routines: Каждый вечер перед сном я делаю лёгкую растяжку…
Every evening before bed I do a light stretch…

If you said я сделаю лёгкую растяжку, that would be:

Future, perfective: I will do a light stretch (once / at some specific time).

For describing a regular bedtime routine, делаю (imperfective present) is the natural choice.

Can I change the word order, for example: Я перед сном делаю лёгкую растяжку спины и плеч?

Yes, Russian word order is fairly flexible. All these are grammatically correct:

Перед сном я делаю лёгкую растяжку спины и плеч.
Я перед сном делаю лёгкую растяжку спины и плеч.
Я делаю перед сном лёгкую растяжку спины и плеч.

Differences are mostly about rhythm and emphasis:

Starting with Перед сном focuses on the time: Before bed, I…
Starting with Я focuses on the subject: I, before bed, do…

The original word order is very natural and typical in written and spoken Russian.

How would you say “before going to bed” in another, slightly more explicit way in Russian?

A common alternative is:

Перед тем как лечь спать, я делаю лёгкую растяжку спины и плеч.
- literally: Before (the moment) of lying down to sleep, I do a light stretch of the back and shoulders.

Other close variants:

Прежде чем лечь спать, я делаю лёгкую растяжку…
Перед тем как идти спать, я делаю лёгкую растяжку… (a bit less precise)

But перед сном is shorter and completely natural, especially for describing daily routines.

How is this sentence pronounced, and where is the stress?

Stresses (in caps) and approximate pronunciation:

Перед сном я ДЕлаю ЛЁгкую растЯжку спИны и плЕч.

Syllable by syllable:

перед – PE-red (stress on PE)
сном – snom (one syllable)
я – ya
делаю – ДЕ-la-yu (stress on ДЕ)
лёгкую – ЛЁХ-ku-yu (stress on ЛЁ, гк sounds like хк)
растяжку – ra-СТЯЖ-ku (stress on СТЯЖ)
спины – spa-НЫ (stress on НЫ)
и – ee
плеч – plech (one syllable, like “plech”)

Natural rhythm:

Перед сном / я ДЕлаю / ЛЁгкую растЯжку / спИны и плЕч.