Learn Hardcore Ukrainian: Спочатку м’язи болять, але з часом вони стають сильнішими і витривалішими. - At first the muscles hurt, but over time they become stronger and more enduring.

спочатку = at first, initially (more adverb-like, about the phase of a process in general)
- Спочатку м’язи болять… = At first, the muscles hurt…
на початку = at the beginning (of something specific) and usually needs a noun:
- на початку тренування – at the beginning of the training
- на початку тижня – at the beginning of the week

Just На початку м’язи болять without a following noun is possible, but it sounds a bit incomplete or literary, like “At the beginning (of it all), the muscles hurt”.
For everyday speech about a process over time, спочатку is the most natural choice here.

Why is м’язи plural? Could I say the singular м’яз instead?

In Ukrainian, м’язи (plural) is normally used when you talk about muscles in the body generally.

м’яз = one specific muscle
м’язи = muscles, the muscular system

Since the sentence refers to muscles in general (after exercise, all the muscles, not just one), the plural м’язи is natural and expected.

Saying Спочатку м’яз болить would mean one particular muscle hurts (for example, a single muscle you injured), not the typical post-workout soreness.

What case is м’язи in, and why?

М’язи is in the nominative plural.

Dictionary form: м’яз (nominative singular)
Nominative plural: м’язи

Here, м’язи is the subject of the verb болять:

м’язи (що роблять?) болять – the muscles (what do they do?) hurt

Subjects are normally in the nominative case, which is why м’язи is nominative here.

Why is the verb болять used, and why is it in the plural?

The verb is боліти – to hurt / to ache.

Its present tense forms include:

він / вона / воно болить – it hurts
вони болять – they hurt

Since the subject м’язи is plural (вони), the verb must also be plural:

м’язи болять – the muscles hurt

Ukrainian often uses боліти with the hurting body part as the subject:

Спина болить. – My back hurts.
Очі болять. – My eyes hurt.

You can also make it more personal:

У мене болять м’язи. – My muscles hurt.
(literally: At me the muscles hurt)

What does з часом literally mean, and why is часом in that form?

З часом is a fixed expression meaning “with time / over time / as time passes.”

з is a preposition that often takes the instrumental case.
час (time) in the instrumental singular is часом.

So:

час (nominative) → з часом (with time, over time)

You would not say з час here; the instrumental часом is required by the preposition з in this meaning.

Compare:

через час – in an hour / after some time (a point in the future)
з часом – over time (gradual change)

Here з часом fits perfectly because we are describing a gradual process of getting stronger.

Why do we need вони in але з часом вони стають…? Could we omit it?

Вони = they and refers back to м’язи.

але з часом вони стають сильнішими…
= but over time they become stronger…

In Ukrainian, personal pronouns like вони are often optional when the subject is clear from context or the verb ending. Here, you could say:

…але з часом стають сильнішими і витривалішими.

This is still understandable (the subject is still “the muscles”), and it sounds normal in speech.

Using вони just makes the sentence a bit more explicit and clear, especially in a stand‑alone sentence or in writing.

Why is стають used instead of something like є? What nuance does стають have?

Стають is the 3rd person plural present of ставати – to become.

стають сильнішими – become stronger

This verb expresses a change of state over time, not just a static description.

If you used є (are), it would simply state a quality, not a process:

вони є сильними – they are strong (no idea how they got that way)

Using стають emphasizes:

a process (they are getting / becoming stronger),
a gradual development (linked naturally with з часом – over time).

So стають is the correct and more meaningful choice here.

Why are сильнішими and витривалішими in that form? What case are they?

Both сильнішими and витривалішими are:

instrumental plural forms of comparative adjectives.

Base forms:

сильний – strong
витривалий – enduring, resilient

Comparative masculine singular:

сильніший
витриваліший

Instrumental plural (used here):

сильнішими
витривалішими

In Ukrainian, after verbs that mean become / turn into / be in a new state (like стати / ставати, sometimes бути), you often use the instrumental case for the new quality:

Вони стають сильнішими.
Вона стала лікарем. – She became a doctor.

So the form сильнішими / витривалішими is required by the verb стають expressing a change of state.

How are the comparatives сильнішими and витривалішими formed? Could I instead say більш сильними?

Formation:

Start from the positive adjective:
- сильний – strong
- витривалий – enduring
Form the comparative stem:
- сильніший – stronger
- витриваліший – more enduring
(suffix ‑іш‑ is key in forming the comparative)
Decline that comparative adjective in the needed case and number:
- comparative stem сильніш‑
  - instrumental plural ending ‑ими → сильнішими
- comparative stem витриваліш‑
  - ‑ими → витривалішими

Yes, you can also use analytic forms:

більш сильними і більш витривалими

This is grammatically correct, but:

the synthetic comparatives (сильнішими, витривалішими) are shorter and more natural here.
using більш with these particular adjectives is usually reserved for special emphasis or contrast.

Why do both adjectives repeat the full ending: сильнішими і витривалішими? In English I’d say “stronger and more enduring” without repeating “more”.

In Ukrainian, each coordinated adjective normally carries its full set of endings to agree with the noun in case, number, and (if relevant) gender.

сильнішими – instrumental plural, agreeing with м’язи
витривалішими – instrumental plural, also agreeing with м’язи

You cannot shorten it to something like:

✗ сильнішими і витриваліші
or
✗ сильнішими і витриваліше

Both must match м’язи fully: сильнішими і витривалішими.

So, unlike in English where you can drop more/-er in the second adjective, Ukrainian repeats the full inflected form.

Could the word order be Спочатку болять м’язи, але вони з часом стають сильнішими…? Does that change the meaning?

Yes, you can say:

Спочатку болять м’язи, але вони з часом стають сильнішими і витривалішими.

Both

Спочатку м’язи болять… and
Спочатку болять м’язи…

are grammatically correct. The difference is subtle emphasis:

м’язи болять – slightly more neutral: subject first, then verb.
болять м’язи – can put a bit more emphasis on the fact of hurting (the verb), then specify what hurts.

In everyday speech, both orders sound natural; the overall meaning is the same.

What is the apostrophe in м’язи for, and how does it affect pronunciation?

The sign ’ in м’язи is the Ukrainian apostrophe.

It shows that:

the consonant before it (м) is not softened by the following я, and
я starts a new syllable with a [j] sound (like y in yes).

So:

м’ясо – [mʲa-so] with a clear [m] + [j]a sequence
м’язи – [mʲa-zy] (approximately m-ya-zy), not мязи with a softened consonant and no [j].

The apostrophe appears:

after certain consonants (б, п, в, м, ф, р)
before я, ю, є, ї
to keep the [j] sound separate and prevent softening.

So м’язи is written with an apostrophe to indicate м + я pronounced as two distinct sounds.