Impetus hostium magnus est.

Impetus is nominative singular, and it is the subject of est.

A beginner might hesitate because impetus does not look like a typical 2nd-declension nominative such as servus. That is because impetus is a 4th-declension noun, not a 2nd-declension one.

In this sentence, you can tell it is nominative singular because:

• it is the thing being described
• magnus agrees with it
• est is singular

So impetus ... magnus est means that the impetus/attack/charge is great.

Hostium is genitive plural, meaning of the enemies or of the enemy.

The genitive here shows whose impetus it is. In other words, it answers the question the attack/charge of whom? → of the enemies.

So the structure is:

• impetus = attack, charge, assault, momentum
• hostium = of the enemies

Together: the attack of the enemies or more naturally the enemy's attack.

Because it comes from hostis, meaning enemy, and hostium is its genitive plural form.

A native English speaker may wonder why the Latin has a plural when English might say the enemy. Latin often uses a plural where English may use:

• the enemies
• the enemy
• enemy forces

So hostium is grammatically plural, even if the most natural English translation might use a collective singular.

Because magnus agrees with impetus.

In Latin, adjectives must agree with the nouns they describe in:

• gender
• number
• case

Here:

• impetus is masculine
• impetus is singular
• impetus is nominative

So the adjective must also be:

• masculine
• singular
• nominative

That gives magnus.

Because impetus is masculine, not neuter.

A learner might expect magnum because English does not show grammatical gender. But in Latin:

• magnus = masculine nominative singular
• magna = feminine nominative singular
• magnum = neuter nominative singular

Since impetus is a masculine noun, the correct form is magnus.

Here it is singular.

This is a very good question, because some 4th-declension forms can look identical in writing. The clearest clue in this sentence is magnus est:

• magnus = singular
• est = singular

So impetus hostium magnus est means the attack/charge of the enemy is great, not the attacks are great.

It is a sentence with a linking verb:

• impetus hostium = subject
• est = linking verb
• magnus = predicate adjective

So Latin is doing something very similar to English in a sentence like:

• The attack is great

The adjective magnus does not directly modify the noun inside the noun phrase in the way a great attack would. Instead, it is linked by est: the attack is great.

Yes, very often.

Latin frequently leaves out forms of esse when they are easy to understand from context. So you might also see:

Impetus hostium magnus.

This would still mean the same thing. However, with est included, the sentence is fully explicit and very standard.

Yes, Latin word order is flexible.

This sentence could be rearranged in several ways without changing the basic meaning, for example:

• Magnus est impetus hostium
• Hostium impetus magnus est
• Impetus magnus est hostium

The usual reason for changing the order would be emphasis or style, not grammar.

In the given version:

• impetus hostium stays together as a noun phrase
• magnus est finishes the statement neatly

So the order is natural and straightforward.

Impetus can mean several related things, depending on context, such as:

• attack
• charge
• assault
• rush
• force
• impetus

In a military context with hostium, it often suggests something like the enemy's attack or the enemy's charge.

So even if you already know the overall translation, it is useful to know that the noun has a range of meanings centered on violent motion or driving force.

You would usually look them up as:

• impetus, impetūs
  • attack, charge, impulse
• hostis, hostis
  • enemy
• magnus, magna, magnum
  • great, large
• sum, esse
  • to be

This is helpful because the sentence contains inflected forms:

• impetus = nominative singular of impetus
• hostium = genitive plural of hostis
• magnus = nominative singular masculine of magnus
• est = 3rd person singular of sum

Yes, that is a good way to think about it.

Hostium can be understood as a genitive showing association or possession:

• impetus hostium = the enemies' attack

In context, it is not necessarily ownership in the strict sense. It simply tells you that the impetus/attack belongs to or comes from the enemies.

So for a learner, the enemies' attack is a very useful way to understand the grammar.