Si ad angulum venis, noli statim vertere, sed per viam rectam ambula.

Angulum is accusative singular. It is accusative because it follows the preposition ad, and ad takes the accusative case.

So:

• ad = to, toward
• angulum = corner in the accusative

Together, ad angulum means to the corner or up to the corner.

Venis is 2nd person singular present indicative of venire: you come.

Latin often uses the present tense in conditions like this when giving general instructions:

• Si ad angulum venis... = If you come to a corner...

In English, we also often use the present in the if-clause for this kind of statement:
If you come to a corner, don’t turn...

So this is normal Latin usage, not a mistake or an odd simplification.

Venis comes from the verb venire, meaning to come.

Its form here is:

• venis = you come
• person: 2nd
• number: singular
• tense: present
• mood: indicative

So the speaker is talking to one person: if you come...

This is one of the standard ways Latin makes a negative command.

• noli = do not, literally something like be unwilling
• vertere = to turn

So noli vertere means don’t turn.

This construction is very common in Latin:

• noli + infinitive = negative command to one person
• nolite + infinitive = negative command to more than one person

Examples:

• noli currere = don’t run
• nolite currere = don’t run (to several people)

Because after noli, Latin normally uses the infinitive.

So:

• noli vertere = don’t turn
• not a direct imperative form of vertere, but part of the noli + infinitive pattern

This may feel strange to an English speaker, but it is a standard Latin structure.

Statim means immediately, at once, or right away.

It is an adverb, so it modifies the verb vertere:

• noli statim vertere = do not turn immediately

Its position before vertere is natural Latin word order. Latin word order is flexible, but adverbs are often placed near the word they modify.

Sed means but. It introduces a contrast:

• noli statim vertere = don’t turn immediately
• sed per viam rectam ambula = but walk along the straight road

So the sentence first tells you what not to do, then what to do instead.

Viam is accusative singular because it follows the preposition per, and per takes the accusative.

• per = through, along
• viam = road/street in the accusative

So per viam means along the road or through the street.

Rectam is an adjective modifying viam, so it must agree with it in:

• gender
• number
• case

Since viam is:

• feminine
• singular
• accusative

rectam must also be:

• feminine
• singular
• accusative

So:

• viam rectam = the straight road

It can be understood as along the straight road in this context.

The basic meaning of per is through, but in directions and movement it often naturally comes across in English as along or by way of. So a smooth translation is:

• per viam rectam ambula = walk along the straight road

Ambula is the 2nd person singular present imperative of ambulare, meaning walk!

So it is a command addressed to one person.

Compare:

• ambula = walk! (to one person)
• ambulate = walk! (to more than one person)

Because Latin often forms:

• positive commands with the imperative
  
  • ambula = walk!
• negative commands with noli + infinitive
  
  • noli vertere = don’t turn!

So the sentence combines a negative command and a positive one:

• don’t turn immediately
• but walk along the straight road

Yes, via can mean road, street, or way, depending on context.

In a sentence giving directions like this, English might use either:

• road
• street
• sometimes even route

So per viam rectam ambula could be understood as walk along the straight street/road.

Latin word order is much more flexible than English word order because Latin uses endings to show grammatical relationships.

For example, in this sentence:

• ad tells you that angulum is governed by a preposition
• per tells you that viam rectam is a prepositional phrase
• ambula is clearly the command
• noli vertere is clearly the negative command

Because the endings and forms do so much work, Latin can arrange words for emphasis or style more freely than English can. This sentence is still quite straightforward and natural Latin.

You would change the singular verb forms to plural ones:

• venis → venitis
• noli → nolite
• ambula → ambulate

So the plural version would be:

Si ad angulum venitis, nolite statim vertere, sed per viam rectam ambulate.

That means: If you come to a corner, don’t turn immediately, but walk along the straight road.