Si quis avarus et invidus est, amicos facile amittit; si liberalis et sincerus est, amicitiam diutius servat.

Because after words like si (if), Latin often uses quis as an indefinite pronoun, not an interrogative one.

So:

• si quis ... est = if anyone is ...
• not if who is ... ?

This is a very common pattern in Latin:

• si quis = if anyone
• nisi quis = unless someone / unless anyone
• ne quis = lest anyone

So here quis is not asking a question; it means any person.

They are nominative singular adjectives.

They go with the understood subject quis and are used with est, so they are predicate adjectives:

• quis avarus et invidus est = if anyone is greedy and envious
• si liberalis et sincerus est = if he/she is generous and sincere

In other words, these adjectives are not direct objects; they describe the subject.

Because they are different nouns with different numbers, but both are in the accusative case as direct objects.

• amicos = accusative plural of amicus = friends
• amicitiam = accusative singular of amicitia = friendship

They are accusative because they receive the action of the verbs:

• amicos ... amittit = he loses friends
• amicitiam ... servat = he preserves friendship

Latin is making a slight shift in focus:

• a bad character causes someone to lose friends
• a good character helps someone preserve friendship

This is not a grammatical problem; it is a stylistic and meaningful choice. The first clause emphasizes the people affected, while the second emphasizes the relationship itself.

English does the same kind of thing:

• A selfish person loses friends; a good person keeps friendship longer.

Facile is an adverb, meaning easily.

It modifies the verb amittit:

• amicos facile amittit = he easily loses friends

Adverbs do not agree with nouns, so facile stays the same. It is not an adjective here.

Compare:

• adjective: facilis = easy
• adverb: facile = easily

Diutius means for a longer time or simply longer.

It is an adverb modifying servat:

• amicitiam diutius servat = he keeps friendship longer

It is the comparative adverb, related to diu (for a long time).

So the contrast is:

• facile = easily
• diutius = longer

Latin often uses a comparative where English also can use one without stating both sides explicitly.

So diutius servat means:

• keeps it longer
• that is, for a longer time

The comparison is understood from the idea of the sentence: a generous and sincere person keeps friendship longer than a greedy and envious person would.

All the verbs are in the present indicative:

• est
• amittit
• est
• servat

This gives a general truth or habitual statement:

• If anyone is greedy and envious, he loses friends easily.
• If he is generous and sincere, he keeps friendship longer.

So this is not about one specific event in the future or past. It is a general observation about human behavior.

Latin often leaves out a repeated subject when it is easy to understand from context.

So after:

• si quis avarus et invidus est ...

the second half naturally continues with the same kind of implied subject:

• si liberalis et sincerus est ...

The sense is:

• if anyone is greedy and envious ...
• if that same sort of person is generous and sincere ...

Latin likes this kind of omission when the structure is clear.

Yes, but it is also very normal Latin.

The sentence is neatly balanced:

• Si quis avarus et invidus est, amicos facile amittit
• si liberalis et sincerus est, amicitiam diutius servat

A few things to notice:

• the si clause comes first
• the adjectives are grouped together
• the direct object comes before the verb
• the main verb comes at or near the end of each clause

This gives the sentence a very parallel, proverb-like rhythm. Latin word order is flexible, and here the arrangement helps the contrast stand out clearly.

No. In this sentence, liberalis means generous, open-handed, or noble in giving.

That is a very common thing for English speakers to wonder, because the English word liberal has strong modern political meanings. In Latin, the basic sense is about generosity and character, not politics.

It is very close, but in Latin it can carry a slightly broader sense such as genuine, honest, straightforward, or unmixed/pure depending on context.

Here sincerus most naturally means something like:

• genuine
• honest
• sincere

So the sentence is praising a person whose character is both generous and genuine.

Yes. Invidus means envious or resentful, and in some contexts it can be used with a dative of the person or thing envied.

But here you do not need to worry about that, because it is simply a predicate adjective:

• quis ... invidus est = someone is envious

So in this sentence it is just describing the subject, alongside avarus.