Im Mai braucht mein Enkel nichts zu tragen, weil die Großen alles vorbereiten.

Im is a contraction of in dem.

With months, German commonly uses in + dative for the meaning in a certain month:

• im Mai = in May
• im Januar = in January

So:

• in dem Mai → im Mai

This is the normal idiomatic form.

German main clauses follow the verb-second rule. That means the finite verb must be in the second position.

Here, Im Mai is placed first for time emphasis, so the verb comes next:

• Im Mai | braucht | mein Enkel ...

If you started with the subject, it would be:

• Mein Enkel braucht im Mai nichts zu tragen.

Both are correct. The version in your sentence just puts the time expression first.

Here brauchen means to need in the sense of to have to or to be required to do something.

A very common pattern is:

brauchen + nicht/nichts + zu + infinitive

So:

• Er braucht nichts zu tragen. = He doesn’t need to carry anything.

This structure is especially common in negative statements and questions:

• Du brauchst nicht zu kommen. = You don’t need to come.
• Brauche ich das zu machen? = Do I need to do that?

Because nichts means nothing, while nicht is the general word not.

Compare:

• Er trägt nichts. = He carries nothing.
• Er trägt nicht. = He is not carrying. / He does not carry.
  (This is grammatically possible, but it means something different.)

In your sentence:

• nichts zu tragen literally means to carry nothing

In natural English, that often becomes not to have to carry anything.

Because tragen is an infinitive linked to brauchen, and in German these infinitive constructions usually go to the end of the clause.

So the structure is:

• braucht = finite verb
• nichts zu tragen = infinitive phrase

This is normal German word order:

• Ich brauche das nicht zu machen.
• Sie braucht nichts zu sagen.

Tragen can mean several things, most commonly:

• to carry
• to wear

Which meaning fits depends on context.

So nichts zu tragen could mean:

• nothing to carry
• nothing to wear

If the meaning has already been shown to the learner, that context tells you which one is intended here. German itself often leaves this open until the context makes it clear.

Because weil introduces a subordinate clause, and in German subordinate clauses are separated by a comma.

So:

• ..., weil die Großen alles vorbereiten.

This is standard punctuation in German.

Weil means because, and it sends the conjugated verb to the end of its clause.

Compare:

Main clause:

• Die Großen bereiten alles vor.

After weil:

• ..., weil die Großen alles vorbereiten.

So the verb that would normally be in second position moves to the end of the subordinate clause.

Vorbereiten is a separable verb:

• base form: vorbereiten
• main clause: Sie bereiten alles vor.

In a subordinate clause, the separated prefix and the verb come back together:

• ..., weil sie alles vorbereiten.

So:

• main clause: Die Großen bereiten alles vor.
• subordinate clause: ..., weil die Großen alles vorbereiten.

That is why you see vorbereiten as one word at the end.

Die Großen is a nominalized adjective, meaning something like:

• the big ones
• the older ones
• the grown-ups

The exact meaning depends on context. It could refer to:

• older children
• adults
• the bigger people in a family or group

German often uses adjectives this way:

• die Kleinen = the little ones
• die Alten = the old people / the elderly
• die Jungen = the young ones / boys / young people

Because it is being used like a noun.

Normally the adjective is lowercase:

• große Kinder = big children

But when the adjective stands on its own as a noun-like word, it is capitalized:

• die Großen = the big ones / the grown-ups

This is called a nominalized adjective.

Mein Enkel is in the nominative case because it is the subject of braucht.

You can tell it is the subject by the sentence structure and the verb agreement:

• mein Enkel braucht = my grandson needs

So:

• mein Enkel = nominative subject
• nichts = object-like element meaning nothing

Because die Großen refers to more than one person.

Here:

• die Großen = plural subject
• vorbereiten = plural verb form

So the clause matches:

• die Großen ... vorbereiten

If it were singular, it would be something like:

• der Große bereitet alles vor. = The big one / the grown-up prepares everything.