Bigae et quadrigae inter se colliduntur.

Because bigae and quadrigae are usually plural-only nouns in Latin.

• bigae literally refers to a two-horse team / a two-horse chariot
• quadrigae literally refers to a four-horse team / a four-horse chariot

Even when Latin is talking about what English would treat as a single chariot, these words are commonly used in the plural form. This is a normal feature of these particular nouns, not a mistake.

So a learner should not be surprised that:

• bigae is plural in form
• quadrigae is plural in form

Latin simply uses those plural forms as the standard dictionary words.

They are nominative plural.

Why?

• They are the subject of the sentence.
• The verb colliduntur is 3rd person plural, so it matches a plural subject.
• The ending -ae in first-declension nouns can be nominative plural.

So:

• bigae = nominative plural
• quadrigae = nominative plural

Together they form the compound subject: bigae et quadrigae.

Because the subject is plural.

The sentence has two subject nouns joined by et:

• bigae
• quadrigae

So Latin uses a plural verb: colliduntur = they collide / they are dashed together.

Even more specifically, the ending -ntur tells you it is:

• 3rd person
• plural
• present
• passive in form

So the basic agreement is exactly what you would expect: plural subject, plural verb.

Yes, -ntur is a passive personal ending.

That is often one of the first things English-speaking learners notice, because in English we would normally say they collide, not they are collided.

Here, Latin uses colliduntur with inter se to express the idea they collide with one another or they crash into each other.

A helpful way to think about it is:

• active-looking English: they collide
• Latin here: they are dashed together / they collide

So the form is passive, but the overall meaning in context is the natural reciprocal idea collide with one another.

Inter se means with one another, among themselves, or between themselves.

Break it down:

• inter = between / among
• se = themselves

So inter se colliduntur means they collide with each other.

This is a very common Latin way to express a reciprocal idea. English often uses:

• each other
• one another

Latin often uses:

• inter se

Because inter takes the accusative case, and se is the accusative form of the reflexive pronoun here.

So:

• inter
  • accusative
• therefore se

The pronoun se refers back to the subject, which is bigae et quadrigae. In other words, the chariots are colliding not with something else, but with one another.

It could, but the meaning would be less explicit.

Without inter se, the sentence would still suggest some kind of crashing or being dashed together, but inter se makes the reciprocal sense very clear:

• colliduntur = they collide / are dashed together
• inter se colliduntur = they collide with each other

So inter se is there to remove ambiguity and to express the idea neatly and idiomatically.

Not very much. Latin word order is more flexible than English word order because the endings show the grammar.

So these all keep basically the same core meaning:

• Bigae et quadrigae inter se colliduntur.
• Inter se bigae et quadrigae colliduntur.
• Bigae et quadrigae colliduntur inter se.

The given order is natural and clear:

• subject first: Bigae et quadrigae
• then the reciprocal phrase: inter se
• verb at the end: colliduntur

That final verb position is especially common in Latin prose.

They can refer to either, depending on context.

Historically and literally:

• bigae = a pair of horses, or the chariot drawn by them
• quadrigae = a team of four horses, or the chariot drawn by them

In many contexts, English simply translates them as:

• two-horse chariots
• four-horse chariots

So if the meaning shown to the learner is about chariots, that is perfectly normal. But it is useful to know that the Latin words are connected to the team arrangement, not just the vehicle in isolation.

A learner will usually meet them as:

• bigae, -arum, f. pl.
• quadrigae, -arum, f. pl.

This tells you several things:

• they are feminine
• they belong to the first declension
• they are normally used in the plural

That last part is especially important, because it explains why the sentence does not use singular-looking forms such as biga or quadriga in ordinary usage.

In a classroom-style Classical pronunciation, roughly:

BEE-guy et kwah-DRI-guy IN-ter seh kol-LID-doon-toor

A few useful points:

• ae is usually pronounced like eye in Classical Latin
• qu is pronounced kw
• c is always hard, as in cat
• vowels matter, so try not to reduce them too much

So:

• bigae ≈ BEE-guy
• quadrigae ≈ kwah-DRI-guy
• colliduntur ≈ kol-LID-doon-toor