Si quis commodum privatum quaerit, bonum commune saepe neglegit.

Si means if and introduces the condition.

So the sentence has two parts:

• Si quis commodum privatum quaerit = If anyone seeks private advantage
• bonum commune saepe neglegit = he often neglects the common good

This is a very common Latin pattern for an if sentence.

This is a classic feature of Latin.

Normally, aliquis means someone / anyone, but after certain words such as:

• si = if
• nisi = unless
• num = whether
• ne = lest

Latin often uses quis instead of aliquis.

So after si, quis means something like anyone or someone.

Quis is nominative singular.

It is the subject of both verbs:

• quaerit = seeks
• neglegit = neglects

So quis is the person doing both actions:

• seeking private advantage
• neglecting the common good

Even though English may say if anyone... he..., Latin just keeps quis as the subject and does not need to repeat it.

They are in the accusative case because they are direct objects.

• quaerit takes a direct object: what does someone seek?
  commodum privatum
• neglegit takes a direct object: what does someone neglect?
  bonum commune

So:

• commodum privatum = accusative singular neuter
• bonum commune = accusative singular neuter

Privatum is an adjective modifying commodum.

Since commodum is:

• neuter
• singular
• accusative

the adjective must agree with it in:

• gender
• number
• case

So privatus, privata, privatum becomes privatum here.

This is standard Latin adjective agreement.

Because commune comes from the third-declension adjective communis, commune.

That adjective has:

• masculine/feminine nominative singular: communis
• neuter nominative singular: commune

Its neuter accusative singular is also commune.

So in bonum commune:

• bonum is neuter singular accusative
• commune agrees with it, so it is also neuter singular accusative

It may look surprising if you expect a second-declension ending like -um, but third-declension adjectives often work differently.

Literally:

• commodum = advantage, benefit, convenience
• privatum = private
• bonum = good
• commune = common, shared, public

So:

• commodum privatum = private advantage or private benefit
• bonum commune = the common good

In English, the common good is the most natural translation of bonum commune.

They are present indicative active, third person singular.

Here the present tense expresses a general truth or habitual idea, not just something happening right now.

So the sentence means something like:

• If anyone seeks private advantage, he often neglects the common good
• or Whenever someone seeks private advantage, he often neglects the common good

This is a very common use of the Latin present tense.

Latin often leaves out subject pronouns because the verb ending already tells you the person and number.

• quaerit = he/she/it seeks
• neglegit = he/she/it neglects

Since quis has already introduced the subject, Latin does not need to say is or another pronoun in the second clause.

English often repeats the subject, but Latin usually does not.

Saepe means often and modifies neglegit.

So it tells you how often the person neglects the common good.

Latin word order is flexible, so saepe could appear in other places too, but here it naturally goes with the second clause:

• bonum commune saepe neglegit = he often neglects the common good

Word order in Latin is usually more flexible than in English because the endings show the grammar.

This sentence is arranged clearly and neatly:

• Si quis sets up the condition
• commodum privatum comes before quaerit
• bonum commune comes before neglegit
• saepe sits near the verb it modifies

A learner can understand the sentence mainly by the endings, not just by the order.

Still, the order helps the style:

• first private advantage
• then common good

That contrast is probably intentional.

Latin has no articles.

That means there is no separate word for:

• a / an
• the

So Latin simply says:

• commodum privatum
• bonum commune

and English has to decide whether to translate them as:

• private advantage
• a private advantage
• the common good

based on context.

Here the common good is the most natural English rendering, even though Latin does not have a word for the.