Лучше не оставлять коляску у двери, если за ребёнком некому присматривать.

Yes. This is a very common Russian impersonal pattern:

• Лучше + infinitive = It is better to...
• Лучше не + infinitive = It is better not to...

So:

• Лучше не оставлять... = It’s better not to leave...

Russian often leaves out это in this kind of sentence. You could think of the full sense as Это лучше — не оставлять..., but in normal speech and writing that would sound unnecessary.

This structure is similar to:

• Лучше подождать. = It’s better to wait.
• Лучше не спорить. = It’s better not to argue.

Because оставлять is the imperfective verb, and here it expresses general advice or a usual recommendation, not one single completed action.

• оставлять = to leave, to be leaving, to leave habitually/in general
• оставить = to leave once, to leave and complete the action

In this sentence, the meaning is something like:

• As a rule, it’s better not to leave the stroller by the door...

So the imperfective fits naturally.

If you said Лучше не оставить..., that would sound wrong in standard Russian, because after лучше не... when giving general advice, Russian normally uses the imperfective.

Compare:

• Лучше не курить здесь. = It’s better not to smoke here.
• Лучше не говорить об этом. = It’s better not to talk about that.

Because коляску is the accusative singular, and it is the direct object of оставлять.

The dictionary form is:

• коляска = stroller / pram / pushchair

But after a transitive verb like оставлять (to leave), the thing being left goes into the accusative:

• оставлять коляску = to leave the stroller

This is a normal first-declension feminine pattern:

• nominative: коляска
• accusative: коляску

Compare:

• Я вижу машину. = I see the car.
• Не бери сумку. = Don’t take the bag.

Here у means by, near, or right next to.

The noun after у takes the genitive case, so:

• дверь = door
• у двери = by the door / at the door

So:

• оставлять коляску у двери = to leave the stroller by the door

Russian often uses у when something is positioned very close to a reference point.

You could also hear:

• около двери = near the door
• возле двери = near the door

But у двери is very natural and common.

Некому means there is no one to... or nobody is available to...

It comes from the pronoun кому (to whom) with не-, and it is used in impersonal constructions with an infinitive.

So:

• некому присматривать = there is no one to supervise / no one to watch

This pattern is very common:

• Мне не с кем поговорить. = I have no one to talk to.
• Нам негде сесть. = We have nowhere to sit.
• Ему нечего делать. = He has nothing to do.
• Здесь некому помочь. = There is no one here to help.

In your sentence, некому does not mean to no one in a literal word-for-word way. It is a fixed Russian way of saying that no suitable person is available for the action.

Because the verb присматривать requires the preposition за plus the instrumental case.

The pattern is:

• присматривать за кем? за чем? = to look after / keep an eye on someone or something

So:

• ребёнок = child
• instrumental singular: ребёнком
• за ребёнком присматривать = to look after the child

This is something you simply have to learn with the verb.

Compare:

• присматривать за ребёнком
• следить за детьми
• ухаживать за больным

Many Russian verbs are learned best together with the preposition and case they govern.

Присматривать means to keep an eye on, to watch over, or to look after. It suggests supervision and care, especially so that something does not happen.

That makes it a very natural choice with a child.

Some comparisons:

• смотреть = to look, to watch
  This is the general verb for looking at something.
• следить за кем-то / чем-то = to monitor, keep track of, keep an eye on
  This can sound more like observing carefully.
• присматривать за кем-то = to supervise, look after
  This often implies responsibility for someone’s safety or well-being.

So in this sentence:

• за ребёнком некому присматривать = there is no one to look after the child / no one to supervise the child

That is more appropriate than just смотреть.

A fairly literal breakdown would be:

• если = if
• за ребёнком = after the child / over the child
• некому = there is no one to
• присматривать = keep an eye / supervise

So, very literally:

• if there is no one to keep an eye on the child

Or even more mechanically:

• if for watching over the child there is no one

That sounds unnatural in English, but it helps explain the Russian grammar.

Because если за ребёнком некому присматривать is a subordinate clause introduced by если (if).

Russian normally separates this kind of clause with a comma:

• Лучше не оставлять коляску у двери, если за ребёнком некому присматривать.

Just like in English:

• It’s better not to leave the stroller by the door, if there’s no one to watch the child.

Russian punctuation is quite regular here: subordinate clauses introduced by words like если, когда, потому что, хотя usually take a comma.

Yes. Russian word order is flexible, although each version can sound slightly different in emphasis.

For example, you could also say:

• Если за ребёнком некому присматривать, лучше не оставлять коляску у двери.

This puts the condition first:

• If there is no one to watch the child, it’s better not to leave the stroller by the door.

The original version starts with the recommendation first and then gives the reason/condition. Both are natural.

Inside the clause, too, Russian allows some movement, but not every version sounds equally neutral. The original:

• за ребёнком некому присматривать

is very standard and natural.

Yes. In many Russian texts, ё is often written as е, so you may also see:

• ребенком

However, the correct pronunciation is still with ё:

• ребёнком

The dictionary form is:

• ребёнок

In careful teaching materials, children’s books, dictionaries, and learner-friendly texts, ё is often written explicitly. In everyday writing, many native speakers omit the dots, but the word is understood from context.

So:

• за ребёнком and за ребенком mean the same thing
• the pronunciation remains za rib-YON-kam

It is formally if, but in practical use it can feel close to when or whenever in a general safety warning or recommendation.

So the sentence means something like:

• If there’s no one to watch the child, it’s better not to leave the stroller by the door.

But the sense can also be:

• Whenever there’s no one to supervise the child, it’s better not to leave the stroller by the door.

This is common with advice and rules: Russian uses если for the condition, while English may interpret it more broadly depending on context.