Si estudio demasiado, no descanso bien.

Because si means if, so the sentence expresses a condition: If I study too much, I don’t rest well.

Cuando means when, and it usually suggests something that happens regularly or at a known time. Compare:

• Si estudio demasiado, no descanso bien. = If I study too much, I don’t rest well.
• Cuando estudio, necesito silencio. = When I study, I need silence.

So here si is the right choice because the speaker is giving a conditional idea.

Spanish often uses the present tense in both parts of a sentence like this when talking about general truths, habits, or likely results.

So:

• Si estudio demasiado = If I study too much
• no descanso bien = I don’t rest well

This is a very common pattern in Spanish for real or possible situations in the present.

Spanish usually leaves out subject pronouns unless they are needed for emphasis, contrast, or clarity.

The ending -o in estudio already tells you the subject is I. The same is true for descanso.

So:

• estudio = I study
• descanso = I rest

You could say Si yo estudio demasiado... but that would sound more emphatic, like If I study too much...

Here demasiado means too much.

It modifies the verb estudio, so it tells us the speaker studies excessively.

• estudio demasiado = I study too much

Be careful not to confuse it with muy, which means very:

• estudio demasiado = I study too much
• estudio muy poco = I study very little

Bien is an adverb, and it means well. It modifies the verb descanso.

• descansar bien = to rest well
• no descanso bien = I don’t rest well

This works like English well after a verb:

• I sleep well
• I don’t rest well

In Spanish, no normally goes directly before the conjugated verb.

So:

• no descanso = I do not rest
• no estudio = I do not study

That is the standard word order in simple negative sentences.

Not exactly.

• descansar = to rest
• dormir = to sleep

Sometimes they overlap in meaning, especially in everyday speech, but they are not identical.

In this sentence, no descanso bien suggests I don’t rest properly / I’m not getting good rest, which is a bit broader than just I don’t sleep well.

The comma separates the if-clause from the main clause:

• Si estudio demasiado, no descanso bien.

This is very common when the conditional part comes first. It helps show the pause and structure clearly.

In short:

• first part: Si estudio demasiado
• second part: no descanso bien

Yes. You can also say:

No descanso bien si estudio demasiado.

It means the same thing: I don’t rest well if I study too much.

The version with si first is often a little clearer and more natural when introducing the condition first.

No, not with the same meaning.

• estudio mucho = I study a lot
• estudio demasiado = I study too much

Mucho just means a large amount.
Demasiado means more than is good, necessary, or healthy.

So in this sentence, demasiado is important because the idea is that overstudying causes a problem.

Not in this form.

With si for real or likely conditions in the present, Spanish uses the indicative, not the subjunctive:

• Si estudio demasiado, no descanso bien.

The subjunctive is not used after si in this kind of present real condition.

Learners sometimes expect the subjunctive after if, but Spanish does not work that way here.

Usually it sounds like a general statement or a repeated experience.

It suggests something like:

• Whenever I study too much, I don’t rest well
• If I study too much, I don’t rest well

So it is not necessarily about one single occasion. It sounds more like a pattern or personal truth.