undongeul hamyeon momi ganghaejyeoyo.

Because 운동하다 means to exercise, and it is built from the noun 운동 + 하다.

So:

• 운동 = exercise
• 운동을 하다 = to do exercise / to exercise

The particle 을 marks 운동 as the object of 하다.

In Korean, many action verbs work this way:

• 공부를 하다 = to study
• 운전을 하다 = to drive
• 운동을 하다 = to exercise

So 운동을 하면 literally means if/when [someone] does exercise.

하면 is the conditional form of 하다.

• 하다 = to do
• 하면 = if one does / when one does

So 운동을 하면 means:

• if you exercise
• when you exercise

In sentences like this, -면 often gives a general truth or result, not just a one-time future possibility. So this sentence sounds like a general statement: If you exercise, your body gets stronger.

Because 몸 is the subject of 강해져요.

The sentence is structured like this:

• 운동을 하면 = if you exercise
• 몸이 강해져요 = the body becomes strong

The verb 강해지다 means to become strong, and the thing that becomes strong is marked with 이/가.

So:

• 몸이 강해져요 = the body becomes strong

It is not 몸을 because the body is not the object of an action here. It is the thing undergoing the change.

강해져요 comes from 강해지다.

Breakdown:

• 강하다 = to be strong
• 강해지다 = to become strong
• 강해져요 = becomes strong / get(s) strong (polite)

More specifically:

1. 강하다 is a descriptive verb meaning to be strong
2. Add -아/어지다 to mean to become
3. 강하다 → 강해지다
4. Then the polite present form is 강해져요

So 강해져요 means becomes strong or gets stronger.

Because the sentence is talking about a change caused by exercise.

• 몸이 강해요 = the body is strong
• 몸이 강해져요 = the body becomes strong / gets stronger

Since exercise leads to improvement, 강해져요 is more natural. It expresses the idea that the body changes as a result of exercising.

In English, we often say:

• Exercise makes your body stronger
• If you exercise, your body gets stronger

Korean often expresses that with become language: 강해지다.

Yes. The person who exercises is not explicitly stated, but it is understood from context.

So 운동을 하면 몸이 강해져요 can mean:

• If you exercise, your body gets stronger
• If one exercises, the body becomes stronger
• When people exercise, their bodies get stronger

Korean often leaves out subjects when they are obvious or not important.

A fully explicit version could be something like:

• 운동을 하면 몸이 강해져요 = natural, general statement
• 사람이 운동을 하면 몸이 강해져요 = if a person exercises, the body gets stronger

But the shorter version is much more natural in everyday Korean.

Yes, very often you can.

Both are common:

• 운동을 하면 몸이 강해져요
• 운동하면 몸이 강해져요

In everyday speech, the object particle 을/를 is often dropped when the meaning is clear. Since 운동하다 is such a common expression, 운동하면 sounds very natural.

The version with 을 is a little more explicit or slightly more careful, but both are correct.

It can feel like both, depending on context.

In this sentence, -면 expresses a general cause-and-result relationship. So in natural English, it could be translated as:

• If you exercise, your body gets stronger
• When you exercise, your body gets stronger

The difference is small here. Because this is a general truth, English often uses if for this kind of statement.

So the Korean does not strongly force one single translation. It just means that exercising leads to this result.

-요 makes the sentence polite.

• 강해져요 = polite
• 강해져 = casual
• 강해집니다 = formal polite
• 강해진다 = plain dictionary-style / neutral written style

So 운동을 하면 몸이 강해져요 is a polite, everyday sentence that you could say in normal conversation.

강해져요 is correct and natural, but there are other possibilities depending on nuance.

For example:

• 몸이 강해져요 = your body becomes strong
• 몸이 튼튼해져요 = your body becomes sturdy/healthy/robust

강하다 focuses more on strength. 튼튼하다 often sounds more like healthy, solid, sturdy, physically robust.

So if the meaning shown to the learner is something like exercise makes your body stronger, 강해져요 fits very well.

Because Korean often contracts forms to sound more natural.

The full combination would historically be:

• 강하다 + -아지다
• 강하여지다

But in modern Korean, this becomes:

• 강해지다

Then the polite present form becomes:

• 강해져요

So 강해져요 is the normal modern form. It is not a special irregularity you need to worry about too much; it is just a common contraction pattern.