Learn Hardcore Latin: Cras puer et puella in horto cum amicis ridebunt. - Tomorrow the boy and the girl will laugh with their friends in the garden.

Putting cras first makes it clear right away that the whole action happens tomorrow.
You could also see it later in the sentence (e.g. Puer et puella cras ridebunt), and the meaning would stay the same; only the emphasis or style changes slightly.

So cras is just giving the time: Tomorrow the boy and the girl will laugh in the garden with friends.

Why is the verb ridebunt plural when there is only one boy and one girl?

Latin verbs agree in number (singular/plural) with the whole subject, not with each individual noun.

puer = boy (singular)
puella = girl (singular)
puer et puella together = they (plural)

So the subject is two people together, which is grammatically plural, and the verb must be plural: ridebunt = they will laugh.

If there were only one subject, e.g. puer cras ridebit, the verb would be ridebit (3rd singular).

How is ridebunt formed, and what exactly does it mean?

The verb is from rideō, ridēre = to laugh.

Stem: ride-
Future tense marker for 2nd conjugation, 3rd person plural: -bunt
ride- + -bunt = ridebunt = they will laugh / they will be laughing (future)

So:

rident = they laugh / are laughing (present)
ridebunt = they will laugh / will be laughing (future)

You recognize the future in 2nd conjugation by endings like -bo, -bis, -bit, -bimus, -bitis, -bunt.

What cases are puer, puella, horto, and amicis, and what roles do they play?

puer – nominative singular, masculine
- Subject: the boy
puella – nominative singular, feminine
- Subject: the girl
hortō – ablative singular, masculine (from hortus)
- Used with in to show location: in the garden
amicīs – ablative plural (gender depends on meaning, often masculine or mixed group)
- Used with cum to show association/companionship: with (their) friends

So the core structure is:

Subject: puer et puella
Place where: in hortō
With whom: cum amicīs
Verb: ridebunt

Why is it in hortō and not in hortum? How do I know it means “in the garden” and not “into the garden”?

Latin in can take ablative or accusative, and the case changes the meaning:

in
- ablative = in / on (location, where?)
  - in hortō = in the garden (where they are)
in
- accusative = into / onto (motion, where to?)
  - in hortum = into the garden (they are going there)

In the sentence, hortō is ablative singular, so in hortō means in the garden, a place where the laughing happens, not a destination.

Why is amicis in the ablative, and what does cum do here?

The preposition cum normally takes the ablative case and expresses accompaniment or association:

cum amicīs = with friends (accompaniment: in whose company?)

Grammar pieces:

amicus, amicī (friend) → ablative plural amicīs
cum
- ablative = with (as in together with)

So cum amicīs tells us who is with the boy and girl when they laugh.

Why doesn’t Latin use a word for “the” like English does?

Classical Latin has no separate word for “the” or “a/an”. The noun’s case ending and the context tell you how to translate it.

puer can mean boy, the boy, or even a boy, depending on context.
Similarly, in hortō can be translated as in a garden or in the garden.

In this sentence, natural English is:
Tomorrow the boy and the girl will laugh in the garden with friends.
But you could also reasonably say:
Tomorrow a boy and a girl will laugh in a garden with friends.

Latin leaves that article detail to the reader.

Why is it puer et puella, not puer puellaque? Do both ways work?

Yes, both constructions are grammatically correct:

puer et puella – the normal, everyday way: the boy and the girl
puer puellaque – more literary/poetic: the boy and the girl, using -que (= and) attached to the second noun.

-que is an enclitic meaning and and is added to the end of the second word:

puer puellaque = puer et puella
Word-for-word: boy girl-and

Your sentence just uses the more typical et.

If puer is masculine and puella is feminine, why is the verb not marked as feminine or something like that?

Latin verbs do not change for gender; they only change for person and number.

ridebunt is 3rd person plural: they will laugh.
It doesn’t matter if they are all males, all females, or a mixed group.

Gender agreement in Latin mainly affects adjectives, participles, and pronouns, not the finite verb endings. So here:

Masculine noun: puer
Feminine noun: puella
Together: a mixed group → still just 3rd person plural, so ridebunt.

Could the sentence have a different word order, like Puer et puella cum amicis in horto cras ridebunt? Would that change the meaning?

Latin word order is relatively flexible because the endings show the roles of words. You could see:

Puer et puella cras cum amicīs in hortō ridebunt.
Cras puer et puella ridebunt in hortō cum amicīs.
In hortō cum amicīs puer et puella cras ridebunt.

All still mean essentially: Tomorrow the boy and the girl will laugh in the garden with friends.

Changing the order can slightly change emphasis or rhythm, but not the basic meaning, because:

puer, puella are nominative → subject
hortō, amicīs are ablative with prepositions in, cum → place and accompaniment
ridebunt remains the verb.

How would the meaning change if it were pueri et puellae in horto cum amicis ridebunt?

Then both pueri and puellae would be plural:

pueri = boys (nominative plural)
puellae = girls (nominative plural)

So the sentence would mean:
Tomorrow the boys and the girls will laugh in the garden with friends.

The verb ridebunt is still correct because it’s 3rd person plural and agrees with a plural subject. The original sentence talks about one boy and one girl; this new version talks about several boys and several girls.