Learn Hardcore Japanese: mainiti undou wo sureba, tairyoku ga sizen ni mi ni tuku to omoimasu. - If you exercise every day, I think you naturally build up your physical strength.

You can also say 毎日運動すれば in casual speech; people will still understand you. But the more standard/neutral expression is 運動をする, so in careful writing or teaching materials you will usually see:

毎日 運動をすれば …

What does ～ば in すれば mean, and how is it different from using したら or すると?

すれば is the -ば conditional form of する.

Roughly, the three common conditionals here feel like this:

毎日運動をすれば、…
→ “If you (in general) exercise every day, then …”
General / hypothetical condition. Often used for rules, tendencies, or advice.
毎日運動をしたら、…
→ “If/when you (actually) exercise every day, then …”
A bit more event‑like / “after that happens” feeling. Often more concrete, tied to a specific situation.
毎日運動をすると、…
→ “When(ever) you exercise every day, … / If you do, (as a rule) …”
Often used for habitual results, cause‑and‑effect, or things that always happen.

In this sentence, すれば fits well because the speaker is giving a general statement or advice about what tends to happen if you keep up daily exercise.

Why is が used with 体力 (体力が), not は or を?

Here, 体力 is the subject of the verb phrase 身につく (“to be acquired / to develop”), so Japanese marks it with が:

体力が身につく
→ “Physical strength gets acquired / develops.”

If you used は, it would change the nuance:

体力は自然に身につく
→ “As for physical strength, it (in particular) will naturally develop.”

That puts contrast or topical emphasis on 体力 (“speaking of physical strength (as opposed to something else)…”).

Using が keeps it as a straightforward subject: “(What) will naturally attach/develop? → 体力が.”

What does 自然に mean here? Is it “naturally” as in “by nature” or “without effort”?

自然に is the adverb form of 自然 (natural), and here it means:

“naturally”
“without forcing it”
“as a natural result”

So in this sentence, it implies:

“your physical strength will naturally develop (as a natural consequence of daily exercise, without you having to force it directly).”

It doesn’t mean “you are naturally strong from birth”; it’s about the process being natural and automatic once the condition (daily exercise) is met.

What exactly does 身につく mean? Literally it looks like “attach to the body.”

Literally, 身につく is:

身 (body / one’s person / oneself)
に (to / on)
つく (to stick / to attach / to come to)

So literally something like “to stick to you / to become part of you.”

Idiomatically, it means things like:

“to acquire (a skill/ability/knowledge)”
“to master / internalize”
“to really take root in you”

Common uses:

日本語の文法が身についてきた。
→ “Japanese grammar has started to really stick / I’m really getting it.”
良い生活習慣が身につく。
→ “You develop good life habits.”

Here, 体力が自然に身につく = your physical strength will naturally become part of you / build up / develop.

What’s the difference between 身につく and 身につける?

They form an intransitive / transitive pair:

身につく (intransitive)
→ “to be acquired / to become internalized”
(Focus on the skill/ability itself coming to you.)
身につける (transitive)
→ “to acquire / to master / to learn (something)”
(Focus on you actively acquiring it.)

Examples:

毎日練習すれば、技術が身につきます。
→ “If you practice every day, the skill will become yours / will stick.”
毎日練習して、技術を身につけました。
→ “By practicing every day, I gained / mastered the skill.”

In your sentence, the focus is on the natural result (“physical strength will come to you”), so the intransitive 身につく is appropriate.

Why is つく (non‑past) used even though the meaning is about the future (“will develop”)?

Japanese does not have a separate “future tense” like English. The non‑past form (つく / 食べる / 行く, etc.) can cover:

present
near future
general truths / tendencies

In a conditional sentence like:

毎日運動をすれば、体力が自然に身につく。

the non‑past in the second clause is usually understood as:

“will (as a general result)”
“tends to”

So it naturally translates as:

“…your physical strength will naturally develop / improve.”

The future meaning comes from the context of the conditional (すれば), not from a special future verb form.

What is the role of と in と思います? Is it “and,” “that,” or something else?

Here, と is the quotative particle. It marks what is being “said,” “thought,” “heard,” etc.

Structure:

[clause] + と + 思います
→ “I think that [clause].”

So:

体力が自然に身につくと思います。
→ “I think (that) your physical strength will naturally develop.”

The clause 体力が自然に身につく is the content of the thought, and と marks it as such. It does not mean “and” here; it functions more like English “that” in “I think that…”.

Why is it 思います (polite) when すれば and つく are in plain form? Is this mixing okay?

Yes, this is normal and completely fine.

In Japanese, the overall politeness level is decided by the main verb at the end of the sentence. Subordinate clauses often appear in plain form, even in polite speech.

Here:

毎日運動をすれば、… (conditional clause, plain form)
体力が自然に身につく (embedded/clausal content, plain form)
…と思います。 (main predicate, polite form)

Because 思います is in the polite -ます form, the whole sentence counts as polite speech. This pattern is very common:

毎日運動をすれば、体力が自然に身につくと思います。

Who is the subject of 思います here? There is no “I” written.

In Japanese, subjects and objects are often omitted when they are clear from context.

With ～と思います, unless otherwise specified, the subject is almost always “I”:

（私は）そう思います。
→ “(I) think so.”

So the full, fully explicit version of your sentence would be:

私は、毎日運動をすれば、体力が自然に身につくと思います。

But Japanese usually leaves out 私は because it’s obvious who is doing the “thinking” in a statement like this.

Can the word order around 毎日 and 運動を change? For example, is 運動を毎日すれば okay?

Yes, Japanese word order is relatively flexible for adverbs like 毎日.

These are all grammatically possible:

毎日運動をすれば、…
運動を毎日すれば、…
毎日、運動をすれば、…

Nuance:

毎日運動をすれば
→ Feels very neutral and natural: “If you exercise every day…”
運動を毎日すれば
→ Slightly more emphasis on 運動, like “If it’s exercise that you do every day…”. Still fine, just a tiny shift in focus.

The original order (毎日運動をすれば) is the most common/neutral way to express the idea.

Why is there a comma (、) after すれば? Is it required?

The comma after すれば just marks the pause between the condition and the result:

毎日運動をすれば、｜体力が自然に身につくと思います。

It is:

not grammatically required, but
very common and recommended, because it improves readability.

You could also write it without the comma:

毎日運動をすれば体力が自然に身につくと思います。

Both are correct; the meaning does not change.