Si occasio bona adest, ea non neglegenda est.

They are both nominative singular feminine.

• occasio = opportunity, a feminine noun
• bona = good, an adjective agreeing with occasio

Since occasio is the subject of adest, it is in the nominative case, and the adjective has to match it in gender, number, and case.

So:

• occasio = nominative singular feminine
• bona = nominative singular feminine, modifying occasio

Adest is the verb adesse, meaning to be present, to be at hand, or to be available.

It is made from:

• ad- = to, toward, at
• est = he/she/it is

So adest literally means something like is present or is at hand.

In this sentence, occasio bona adest means that a good opportunity is present / is available / has come up.

Ea means that one or it, and it refers back to occasio bona.

Since occasio is feminine singular, the pronoun referring to it must also be feminine singular nominative:

• ea = nominative singular feminine

Latin often uses a demonstrative or pronoun where English might simply say it.

So:

• occasio bona = the good opportunity
• ea = it / that opportunity

Not exactly. Ea can function in different ways depending on context.

Here it is not really she in a personal sense. It is better understood as:

• it
• that
• that opportunity

Latin does not always separate she and it the way English does. Because occasio is grammatically feminine, the pronoun referring to it is feminine too, even though in English opportunity is it, not she.

This is a passive periphrastic, also called the gerundive of obligation.

It consists of:

• neglegenda = gerundive of neglegere (to neglect)
• est = is

So literally it means:

• is not to be neglected
• must not be neglected
• should not be neglected

This construction expresses necessity, duty, or something that ought to be done.

Neglegenda is a gerundive, feminine nominative singular, agreeing with ea (and therefore with occasio).

Its ending is feminine nominative singular because the thing that must not be neglected is the opportunity, which is feminine singular.

So:

• ea = feminine singular nominative
• neglegenda = feminine singular nominative

They match because neglegenda is describing what must happen to ea.

Latin often leaves that unstated when it is general or obvious from context.

In a passive periphrastic, the person who has the obligation can be expressed, often with the dative of agent, for example:

• nobis non neglegenda est = it must not be neglected by us / we must not neglect it

But if the sentence is making a general point, Latin can simply say:

• ea non neglegenda est = it must not be neglected

without naming the person responsible.

Because there are really two separate clauses, and each has its own verb:

1. Si occasio bona adest
   = If a good opportunity is present / arises

2. ea non neglegenda est
   = it must not be neglected

So the first est is built into adest, and the second est completes the passive periphrastic.

Yes. Latin word order is much more flexible than English word order because the endings show the grammatical relationships.

This sentence is arranged in a clear and natural way:

• Si introduces the condition
• occasio bona gives the subject of the first clause
• adest finishes that clause
• ea picks up the idea again in the second clause
• non neglegenda est gives the main statement about it

A different order could still be grammatical, but this version is neat and easy to follow. Latin often places an important verbal idea at the end of a clause, so adest and est are both in strong positions.

Because this is a simple present condition: a general statement about what should happen whenever the situation occurs.

• Si occasio bona adest = If a good opportunity is present / if a good opportunity comes up
• ea non neglegenda est = it must not be neglected

Latin uses the present indicative here just as English can use the present in general truths or practical advice:

• If the chance comes, don’t miss it.

Yes. Latin neglegere is the source of English neglect.

That can help you remember the phrase:

• neglegenda est = is to be neglected
• non neglegenda est = is not to be neglected

So the sentence has a very transparent connection to English vocabulary.

Yes, Latin sometimes omits a repeated pronoun if the meaning is clear. But including ea makes the reference explicit and tidy:

• Si occasio bona adest, non neglegenda est.
  = possible, with occasio understood

• Si occasio bona adest, ea non neglegenda est.
  = more explicit: if a good opportunity arises, that opportunity must not be neglected

The pronoun helps link the two clauses clearly.