Learn Hardcore Latin: Multi amici per viam lente ambulant. - Many friends walk slowly along the road.

amici is nominative plural masculine, so it is the subject of the verb ambulant.
multi is an adjective describing amici, so the phrase multi amici means many friends and together they are the subject: Many friends walk…

Why is it multi amici and not multi amicos?

Because amici is the subject of the sentence, and in Latin the subject is normally in the nominative case.

amici = nominative plural → friends as the doers of the action.
amicos would be accusative plural, which is used mainly for direct objects (the thing receiving the action).

Since the friends are doing the walking (not being walked by someone), amici (nominative) is correct.

How does multi “agree” with amici?

In Latin, adjectives must “agree” with the nouns they modify in:

Case (here: nominative)
Number (here: plural)
Gender (here: masculine)

So:

amici = nominative plural masculine
multi = nominative plural masculine form of multus

Because all three features match, multi correctly agrees with amici, giving multi amici = many friends.

If the noun were feminine plural nominative (amicae, female friends), the adjective would change too: multae amicae.

What does per viam mean exactly, and why is viam in that form?

Per viam means through the road or, more naturally, along the road.

per is a preposition meaning through, along, or by way of.
per always takes the accusative case.
via (road, street, way) is a first-declension noun; its accusative singular is viam.

So:

via (nominative) = the road (as subject)
viam (accusative) = the road (as object of per)

That is why the phrase must be per viam, not per via.

Could I say in via instead of per viam? Would the meaning change?

Yes, you can say in via, but the meaning is a bit different:

per viam = along the road / through the road → emphasizes movement along a path.
in via = on the road / in the road → emphasizes location (where they are).

So:

Multi amici per viam lente ambulant.
→ Many friends are walking along the road (moving from place to place).
Multi amici in via lente ambulant.
→ Many friends are walking in/on the road (their location is the road; movement is still there, but the path-as-route idea is weaker).

Both are correct Latin; you just choose based on the nuance you want.

What is lente grammatically, and how is it formed?

Lente is an adverb meaning slowly.

It comes from the adjective lentus, lenta, lentum = slow.
A common way to form adverbs from first/second-declension adjectives in Latin is to take the feminine nominative singular form and add -e.

So:

lentus, lenta, lentum (adjective) → slow
lenta (feminine nominative singular) + -e → lente (adverb) = slowly

Thus lente ambulant = they walk slowly.

Where can lente go in the sentence? Must it be before ambulant?

Latin word order is quite flexible. Lente can move around as long as the meaning stays clear:

Multi amici per viam lente ambulant.
Multi amici lente per viam ambulant.
Lente multi amici per viam ambulant.

All of these still mean roughly Many friends walk slowly along the road.
Putting lente right before the verb is common, but not obligatory. Placement can add emphasis:

Lente ambulant slightly emphasizes the manner of walking (slowly).
Multi amici at the start emphasizes the number of friends.

What does ambulant tell us (person, number, tense), and how do we know?

Ambulant is the 3rd person plural present active indicative of ambulare (to walk).

ambul- = the stem (walk)
-a- = the thematic vowel for 1st conjugation
-nt = 3rd person plural ending → they

So ambulant means they walk or they are walking, present tense.

Because the ending -nt already shows they, Latin normally does not need a separate word for they.

Why isn’t there a separate word for they in this sentence?

Latin usually drops subject pronouns (I, you, he, she, they) because the verb ending already shows who is doing the action.

ambulant = they walk
If you wanted a pronoun, ei ambulant (they walk) is possible, but it is usually only added for emphasis or clarity.

In this sentence, multi amici is the subject, so the “they” is understood from ambulant and from the noun phrase:

Multi amici … ambulant.
→ Many friends … (they) walk.

Does the verb always have to go at the end, like ambulant here?

No, it does not have to, but it often does. Latin tends to prefer verb-final order, especially in simple sentences, but word order is flexible and used for emphasis.

All of these are grammatically correct:

Multi amici per viam lente ambulant.
Multi amici lente per viam ambulant.
Per viam lente ambulant multi amici.
Ambulant multi amici per viam lente.

The basic meaning stays the same: Many friends walk slowly along the road.
The default “neutral” order for a simple statement often puts the verb at or near the end.

How would the sentence change if I wanted to say “Many female friends walk slowly along the road”?

You would make the noun and adjective feminine plural:

amica = (female) friend
amicae = nominative plural → female friends
multus, multa, multum (many) → multae in the feminine nominative plural

So:

Multae amicae per viam lente ambulant.
= Many (female) friends walk slowly along the road.

The rest (per viam, lente, ambulant) stays the same.

How would you say “Many friends walk slowly along the roads” (roads in the plural)?

You just make via plural in the accusative:

via, viae (road)
vias = accusative plural → roads (as object of per)

So:

Multi amici per vias lente ambulant.
= Many friends walk slowly along the roads.

Changes:

viam (one road) → vias (several roads)
Everything else stays the same.