Si frenum nimis durum est, equa quoque calcitrare incipit.

Because this is a simple, real condition: If X is the case, Y happens/begins to happen. In Latin, that kind of condition normally uses the indicative, not the subjunctive.

So:

• si ... est = if ... is
• incipit = begins

If Latin wanted a more hypothetical or less direct kind of condition, the verb forms might change, but here the indicative is the normal choice.

Frenum is singular. It is a neuter noun of the second declension.

Here are the relevant forms:

• frenum = the bit / bridle / rein (singular nominative or accusative)
• frena = bits / bridles / reins (plural nominative or accusative)

In this sentence, the subject of est is a single frenum, so the singular is used.

Because durum agrees with frenum, and frenum is neuter singular.

Latin adjectives must agree with the nouns they describe in:

• gender
• number
• case

So:

• frenum = neuter singular
• durum = neuter singular adjective agreeing with it

That is why it is durum, not durus or dura.

Nimis means too much, too, or excessively. Here it modifies durum:

• durum = hard / harsh
• nimis durum = too hard / too harsh

So nimis is an adverb strengthening the adjective.

Because equa specifically means mare, a female horse.

Compare:

• equus = horse, often specifically a male horse
• equa = mare

If the English meaning shown to the learner says mare, then equa is exactly the right word.

Latin quoque usually comes after the word it emphasizes.

So:

• equa quoque = the mare also
• literally, something like the mare too

This is different from English, where also often comes before the word or later in the clause. In Latin, post-position with quoque is very common.

Because incipit means begins, and after verbs like incipio, Latin often uses an infinitive to express the action that begins.

So:

• calcitrare = to kick
• incipit = begins
• calcitrare incipit = begins to kick

If you said calcitrat, that would just mean she kicks / is kicking, without the idea of beginning.

Because -re is the normal ending of the present active infinitive in Latin.

So:

• calcitrat = she kicks
• calcitrare = to kick

After incipit, the infinitive is exactly what Latin expects:

• incipit calcitrare = she begins to kick

No. Calcitrare can be used intransitively, meaning to kick, to kick out, or to start kicking without naming what is being kicked.

So the sentence does not need to say what the mare kicks. The idea is that she starts kicking as a reaction.

No, Latin word order is much more flexible than English word order because the endings show the grammatical relationships.

This sentence has a natural and clear order:

• Si frenum nimis durum est = conditional clause first
• equa quoque calcitrare incipit = main clause after it

But Latin could rearrange parts for emphasis. For example, moving quoque or nimis slightly would change emphasis more than basic meaning.

So the word order here is normal and readable, but not the only possible order.

Equa is nominative singular because it is the subject of incipit.

You can see this from the structure:

• equa = the mare
• incipit = begins

So the mare is the one doing the action of beginning to kick.

Yes. In the clause Si frenum nimis durum est:

• frenum = subject
• est = verb
• durum = predicate adjective describing the subject
• nimis = adverb modifying durum

So the structure is:

• If the bit/bridle is too harsh

This is a very common Latin pattern: noun + adjective + est.