Если на улице мокро, детям лучше разуться у двери, чтобы не испачкать пол.

Мокро here is not a normal adjective modifying a noun. It is a predicative word (sometimes called a “state word”) meaning it is wet / it’s wet outside.

So:

• На улице мокро = Outside, it’s wet
• not a wet street or a wet thing

Compare:

• Улица мокрая = The street is wet
  Here мокрая agrees with улица
• На улице мокро = It is wet outside
  Here мокро describes the general situation or condition

This is very common in Russian:

• На улице холодно = It’s cold outside
• В комнате темно = It’s dark in the room
• Здесь тихо = It’s quiet here

Literally, на улице means on the street, but in many everyday contexts it is best translated as outside.

In this sentence:

• Если на улице мокро = If it’s wet outside

Russian often uses на улице where English would simply say outside, especially when talking about weather or outdoor conditions.

Examples:

• На улице холодно = It’s cold outside
• На улице дождь = It’s raining / There is rain outside

So even though the literal wording is on the street, the natural meaning here is broader: outdoors.

Because Russian often uses the pattern:

• кому + лучше + infinitive

This means something like:

• it is better for someone to do something
• someone had better do something

So:

• детям лучше разуться = the children had better take off their shoes

Here:

• детям = dative plural of дети
• it marks the person for whom the advice or recommendation applies

More examples:

• Мне лучше уйти. = I’d better leave.
• Тебе лучше подождать. = You’d better wait.
• Нам лучше не спорить. = We’d better not argue.

So the dative is not a direct object here; it marks the experiencer or person concerned.

Лучше + infinitive is a very common way to give mild advice or make a recommendation.

So:

• детям лучше разуться = the children had better take off their shoes
• more literally: it is better for the children to take off their shoes

This construction is often softer than a direct command.

Compare:

• Разуйтесь! = Take off your shoes!
  direct command
• Вам лучше разуться. = You’d better take off your shoes.
  advice/recommendation

It is a very natural way to express:

• should
• had better
• it would be better to

Разуться means to take off one’s shoes.

The -ся makes it a reflexive verb, and in this case it implies doing the action to oneself:

• разуть кого-то = to remove someone’s shoes
• разуться = to take off one’s own shoes

So in the sentence:

• детям лучше разуться = the children had better take off their shoes

This verb is specifically about footwear. A more general alternative would be:

• снять обувь = to take off shoes/footwear

But разуться is shorter and very natural in situations like entering a house.

Разуться is perfective because the sentence is talking about a single completed action: taking off the shoes before going in and dirtying the floor.

Perfective verbs in Russian often focus on the result or completion.

Here the idea is:

• the children should take their shoes off at the door

not

• they should be in the process of taking them off
• they should habitually take them off in a general sense

The imperfective partner is разуваться.

Compare:

• Нужно разуться. = You need to take off your shoes.
  one completed action
• Он всегда разувается у двери. = He always takes off his shoes at the door.
  habitual/repeated action

So perfective fits this one specific situation.

У двери means by the door / near the door.

The preposition у usually means at, by, near, and it takes the genitive case.

So:

• дверь → двери after у
• у двери = by the door

This is the natural choice because the children are taking off their shoes near the doorway, not:

• в двери = in the door
• на двери = on the door

Examples:

• стоять у окна = to stand by the window
• ждать у входа = to wait by the entrance
• оставить обувь у двери = to leave shoes by the door

This part expresses purpose:

• чтобы = so that / in order to

So:

• чтобы не испачкать пол = so as not to dirty the floor

The не is there because the goal is to avoid making the floor dirty.

This structure is very common:

• Я закрыл окно, чтобы не было холодно. = I closed the window so that it wouldn’t be cold.
• Он говорил тихо, чтобы не разбудить ребёнка. = He spoke quietly so as not to wake the child.

In your sentence, the purpose of taking off shoes is:

• not to dirty the floor

Испачкать is perfective and means to dirty / to get something dirty as a completed result.

That fits the sentence because it refers to the possible result the speaker wants to prevent:

• чтобы не испачкать пол = so as not to dirty the floor

Why not пачкать?

• пачкать is imperfective and focuses more on the process or repeated action
• испачкать focuses on the completed outcome: the floor ends up dirty

Why пол, not полы?

• пол can mean the floor in a general sense
• полы would mean floors, usually in multiple rooms or as a more concrete plural

Here пол is the normal singular form for the floor of the place they are entering.

Because пол is the direct object of испачкать.

The verb испачкать takes the accusative case, and for an inanimate masculine noun like пол, the accusative singular looks the same as the nominative singular:

• nominative: пол
• accusative: пол

So even though the form does not change, the case is accusative because it is the thing being dirtied.

Compare with a feminine noun:

• испачкать рубашку = to dirty the shirt
  Here the accusative form changes visibly

Yes, from an English-speaking perspective, you can think of an omitted is.

Russian normally does not use a present-tense form of to be in sentences like this.

So:

• На улице мокро literally looks like Outside wet
• but it means It is wet outside

This is completely normal in Russian.

Compare:

• Он дома. = He is at home.
• Я занят. = I am busy.
• Сегодня тепло. = It is warm today.

So there is no missing word in Russian grammar; the present tense to be is simply usually not expressed.

Yes. Russian word order is fairly flexible, and changing it often changes emphasis rather than basic meaning.

The neutral order here is:

• Если на улице мокро, детям лучше разуться у двери, чтобы не испачкать пол.

But you could also hear variations like:

• Если на улице мокро, лучше детям разуться у двери...
• Детям лучше у двери разуться...

These alternatives may sound a little more marked or place emphasis differently, but the core meaning stays the same.

In standard learning contexts, the original order is a good natural model:

1. condition: Если на улице мокро
2. recommendation: детям лучше разуться у двери
3. purpose: чтобы не испачкать пол

Yes, absolutely.

• разуться = to take off one’s shoes
• снять обувь = to remove one’s footwear

Both are correct, but they feel slightly different:

• разуться is compact and very idiomatic for taking off shoes before entering
• снять обувь is a little more explicit and neutral

So these are both natural:

• Детям лучше разуться у двери.
• Детям лучше снять обувь у двери.

In everyday home situations, разуться is especially common.

The understood subject is the same as in the main idea: the children.

Russian often uses чтобы + infinitive when the subject is understood from context.

So:

• детям лучше разуться у двери, чтобы не испачкать пол

means:

• the children had better take off their shoes at the door so that they do not dirty the floor

Russian does not need to repeat они or another subject here.

If the subject were different, Russian would often make that clearer in another way. Here it is naturally understood that the ones taking off their shoes are also the ones who might dirty the floor.