Ne crie pas dans la maison, s'il te plaît.

Because this is an imperative (a command).

In French, the imperative normally drops the subject pronoun:

• (Tu) crie ! → Crie ! (Shout!)
• (Vous) criez ! → Criez ! (Shout! – polite/plural)
• (Nous) crions ! → Crions ! (Let’s shout!)

So Ne crie pas means “(Tu) ne crie pas”, but the tu is omitted, as is standard in commands.

The verb is crier (a regular -er verb).

• Present tense (with subject pronouns):
  
  • je crie
  • tu cries
  • il/elle crie
  • nous crions
  • vous criez
  • ils/elles crient

The imperative uses tu / nous / vous forms, but:

• For -er verbs, the tu form of the imperative drops the final -s:
  • Present: tu cries
  • Imperative: Crie !

So:

• Crie ! = Speak to one person you know well (informal tu).
• Criez ! = Speak to several people or politely to one person (vous).

In the sentence, we’re talking to one familiar person, so it’s Ne crie pas (tu form, imperative).

This is a classic false friend for English speakers.

• crier = to shout / to yell / to cry out (use your voice loudly)
• pleurer = to cry / to weep (with tears)

So Ne crie pas means “Don’t shout / don’t yell”, not “Don’t cry (tears)”.
For “Don’t cry” (tears) you would say Ne pleure pas.

French standard negation uses a two-part structure:

• ne (or n’ before a vowel sound)
• plus a second word like pas, jamais, plus, etc.

They go around the verb:

• Crie ! → Ne crie pas !
  (Don’t shout!)

So the pattern is:

Ne + verb (imperative) + pas

In a normal present-tense sentence it’s the same idea:

• Tu cries. → Tu ne cries pas.
  (You shout. → You don’t shout.)

In everyday spoken French, it’s very common to drop the “ne” and just say:

• Crie pas dans la maison, s’il te plaît.

This is informal but extremely frequent.
In writing, in careful speech, and in exams, you should keep ne: Ne crie pas…

Both exist, but they don’t mean the same thing:

• dans la maison = inside the (particular) house, physically in the building.
  → Focus on the interior space.

• à la maison = at home (where someone lives).
  → Focus on the idea of “home”, not necessarily inside the building at this moment.

So:

• Ne crie pas dans la maison = Don’t shout inside the house (this house).
• Ne crie pas à la maison would sound more like “Don’t shout at home” in general, and is less natural for this very concrete situation.

Here la maison is “the house” in a specific context: usually the house we’re in right now, shared by the speaker and listener (family house, friend’s house, etc.).

• dans la maison = in the house (understood from context)
• dans ma maison = in my house specifically
• dans une maison = in a house (any house)

Also, French almost always uses an article (la, le, les, une, un) with nouns; a bare maison on its own is very limited to special expressions (for example, Maison close, maison mère, etc.). So dans maison would be incorrect here.

s’il te plaît comes from:

• si = if
• il = it (impersonal “it”, referring to what you’re asking)
• te = to you (informal singular object pronoun)
• plaît = pleases (3rd person singular of plaire, “to please”)

Literally: “if it pleases you” → idiomatically: “please”.

Spelling:

• si il contracts to s’il (two vowels together → we drop one i).
• Hence s’il te plaît.

They both mean “please”, but:

• s’il te plaît:

  • informal, singular
  • used with friends, family, children, people you address with tu

• s’il vous plaît:

  • formal or plural
  • used with strangers, people you address with vous, or more than one person

So speaking to a child:

• Ne crie pas dans la maison, s’il te plaît.

Speaking politely to a stranger or to several people:

• Ne criez pas dans la maison, s’il vous plaît.

The comma marks a pause in speech: the main command first, then a politeness tag.

• Ne crie pas dans la maison, s’il te plaît.

You can also put s’il te plaît at the start or end with no comma at the end in informal writing:

• S’il te plaît, ne crie pas dans la maison.
• Ne crie pas dans la maison s’il te plaît.

In normal written French, the comma after the main clause (as in the original) is the most standard-looking.

You change the imperative verb to the vous form:

• Informal, to one person (tu):

  • Ne crie pas dans la maison, s’il te plaît.

• Formal or plural (vous):

  • Ne criez pas dans la maison, s’il vous plaît.

So the sentence adjusts verb and te/vous to match the person and level of formality.