Il suffit que tu changes la couche maintenant pour que le bébé dorme plus calmement.

Il suffit is an impersonal expression meaning it is enough, it is sufficient, or all it takes is.

So Il suffit que... introduces the idea that one thing is enough to make something happen.

Here, Il suffit que tu changes la couche maintenant... means that changing the diaper now is enough to produce the result mentioned afterward.

The il here does not refer to anything specific. It is just the usual grammatical subject in an impersonal expression, like in il faut.

Yes. After il suffit que, French normally uses the subjunctive.

So:

• tu changes here is the subjunctive present of changer
• it happens to look exactly the same in spelling as the indicative present for tu

That can be confusing for learners, because in writing there is no visible difference here. The reason it is subjunctive is the grammar of il suffit que, not the spelling itself.

Because pour que is followed by the subjunctive.

Pour que means so that / in order that, and when French uses a clause with its own subject after it, the verb in that clause goes into the subjunctive.

So:

• pour que le bébé dorme = so that the baby may sleep / so that the baby sleeps

Here dorme is the subjunctive form of dormir.

Use pour when it is followed by an infinitive.

• Je viens pour aider. = I’m coming to help.

Use pour que when there is a new subject + conjugated verb after it.

• Je viens pour que tu m’aides. = I’m coming so that you can help me.

In your sentence, the second part has its own subject, le bébé, so French needs pour que:

• pour que le bébé dorme...

not just pour.

Yes, very often French can use il suffit de + infinitive.

For example:

• Il suffit de changer la couche maintenant pour que le bébé dorme plus calmement.

This version is also natural. It means something like You just need to change the diaper now so that the baby sleeps more peacefully.

The version with que tu changes explicitly says you are the person doing it.
The version with de changer is slightly more general or less directly addressed, even if English would often still translate it with you.

French usually uses an article where English often does not.

So even though English says change the diaper or sometimes just change diapers depending on context, French normally says:

• changer la couche

The definite article la is the standard choice here because it refers to the diaper the baby is wearing.

For the same general reason: French normally likes an article before nouns.

So le bébé is the normal way to say the baby.
Leaving out the article would sound incomplete in standard French.

Maintenant is an adverb, and French often places adverbs in positions that sound natural rhythmically rather than following a fixed English-style rule.

Here:

• tu changes la couche maintenant

is a very natural order.

You may also hear variations depending on emphasis, such as:

• Maintenant, tu changes la couche...
• Tu changes maintenant la couche...

But in everyday French, tu changes la couche maintenant sounds very normal and smooth.

Because the verb dormir is being modified, so French needs an adverb, not an adjective.

• calme = calm, peaceful → adjective
• calmement = calmly, peacefully → adverb

Since the sentence describes how the baby sleeps, French uses:

• dormir plus calmement = sleep more calmly / more peacefully

If you used calme, it would describe a noun, not the action of sleeping.

Literally, it means more calmly.

In natural English, depending on context, you could also understand it as:

• more peacefully
• more soundly
• more quietly

French often uses calmement in situations where English might choose a slightly different adverb for a more natural translation.

Yes. Tu is the familiar singular form of you.

So this sentence is addressed to one person in an informal or familiar situation. That makes sense in a context involving a baby, family member, partner, or close acquaintance.

If you wanted the formal or plural version, you could say:

• Il suffit que vous changiez la couche maintenant pour que le bébé dorme plus calmement.

Notice that with vous, the subjunctive form changes to changiez.

Yes, it is a very common and useful pattern.

It expresses:

• a condition that is enough
• followed by a result or purpose

So the structure is:

• Il suffit que + subjunctive
• pour que + subjunctive

In simpler terms, it means:

• It’s enough for X to happen so that Y happens

This is a good structure to recognize because it appears often in both spoken and written French.