Tout le monde se met en rang.

Because tout le monde is grammatically singular in French.

Even though it refers to many people, it works like everyone in English:

• Everyone is here, not everyone are here.
• So in French: Tout le monde se met..., not Tout le monde se mettent...

That is why the verb is singular: met.

Tout le monde is a fixed expression meaning everyone / everybody.

Here, tout means something like all of, and le monde is treated as a singular noun. So together, tout le monde literally looks like all the world, but idiomatically it means everyone.

Compare:

• tout le monde = everyone
• tous les enfants = all the children
• ils sont tous là = they are all there

So tous is used with plural nouns or plural pronouns, but tout le monde is a special singular expression.

Literally, yes: le monde often means the world.

But in the expression tout le monde, it does not usually mean the planet. It means everybody / everyone.

This is just one of those expressions you learn as a whole:

• le monde = the world
• tout le monde = everyone

The basic verb mettre usually means to put.

When it becomes reflexive, se mettre, it often means to put oneself somewhere or into some state. In many contexts, it becomes more natural in English as:

• to get into
• to start
• to move into
• to line up (in this sentence)

So:

• mettre un livre sur la table = to put a book on the table
• se mettre en rang = to get into line / line up

Here, se mettre is not really translated word-for-word. The natural meaning is to line up.

Because se mettre is a reflexive verb.

In French, reflexive verbs use a reflexive pronoun before the conjugated verb:

• je me mets
• tu te mets
• il/elle/on se met
• nous nous mettons
• vous vous mettez
• ils/elles se mettent

So in Tout le monde se met en rang, se goes before met because that is how reflexive verbs are built.

En rang means in line, in a row, or lined up, depending on context.

In a school setting, se mettre en rang is a very common expression for children forming a line before going into class.

So the whole phrase:

• se mettre en rang = to line up / to get into line

Because en rang is the normal French expression.

French often uses en in set expressions that describe arrangement or position:

• en ligne = in a line / online
• en cercle = in a circle
• en ordre = in order
• en rang = in line

Using dans un rang here would sound unnatural. It would feel too literal, while en rang is the idiomatic phrase French speakers actually use.

Not necessarily. In se mettre en rang, the singular is the normal idiomatic form.

The idea is that everyone is getting into one line or into a lined-up formation, so singular rang works naturally.

You may also see en rangs in some contexts, especially if there are several rows or a more formal/military sense, but en rang is the standard expression for line up.

It is the present tense of se mettre.

Specifically:

• infinitive: se mettre
• present: il/elle/on se met

Since tout le monde is singular, French uses the same verb form as with il/elle/on:

• Tout le monde se met en rang.

Depending on context, the French present can mean:

• Everyone lines up
• Everyone is lining up
• Everyone gets into line

So the exact English translation can vary a little, even though the French form stays the same.