Ianitor viatorem interrogat: "Quid in sacculo portas?"

Because viatorem is the direct object of interrogat.

• viator = traveler
• viatorem = traveler (as the person being questioned)

Latin changes noun endings to show their job in the sentence. Here, the gatekeeper is questioning the traveler, so viator appears in the accusative singular: viatorem.

Interrogat is the 3rd person singular present active indicative of interrogare, meaning he/she/it questions or is questioning.

Here the subject is ianitor = the gatekeeper, which is singular, so the verb must also be singular:

• ianitor interrogat = the gatekeeper questions

For comparison:

• interrogant = they question
• interrogare = to question

Because inside the direct question, the speaker is talking to the traveler.

• portas = you carry
• portat = he/she/it carries

So:

• Quid in sacculo portas? = What are you carrying in the bag?

The gatekeeper is addressing the traveler directly, so Latin uses the 2nd person singular verb form.

Sacculo is in the ablative singular.

It is used after the preposition in to mean in the bag.

A very important Latin pattern is:

• in + ablative = in / on something, indicating location
• in + accusative = into / onto something, indicating motion toward

So:

• in sacculo = in the bag (location)

Quid is the neuter singular form of the interrogative pronoun quis, quid = who? what?

In this sentence, it means what?

• Quid portas? = What are you carrying?

It is neuter because the question is about a thing, not a person.

Latin usually does not use an extra helping verb like English do / does to form questions.

English says:

• What do you carry?
• What are you carrying?

Latin simply says:

• Quid portas?

The verb form itself already tells you the person and number, so no extra do is needed.

Latin word order is much more flexible than English word order because the endings show the grammatical roles.

So both of these are understandable:

• Quid in sacculo portas?
• Quid portas in sacculo?

The given order puts in sacculo before portas, which can slightly emphasize the location: What are you carrying in the bag?

In Latin, word order often affects emphasis more than basic grammar.

Ianitor means doorkeeper, porter, gatekeeper.

It comes from ianua, meaning door.

So ianitor is the person associated with the door or entrance. In context, gatekeeper or porter is often a natural translation.

Classical Latin has no definite article like English the, and no indefinite article like a/an.

So:

• ianitor can mean gatekeeper, a gatekeeper, or the gatekeeper
• viatorem can mean traveler, a traveler, or the traveler
• sacculo can mean bag, a bag, or the bag

You understand which one is meant from the context.

You know from the case endings and the verb form.

• ianitor is in the nominative singular, so it is the subject: the gatekeeper
• viatorem is in the accusative singular, so it is the object: the traveler
• interrogat is singular, agreeing with ianitor

So the sentence means:

• The gatekeeper questions the traveler

not

• The traveler questions the gatekeeper

Either can work, depending on context.

• interrogat literally means questions
• In natural English, it can often be translated as asks

So:

• Ianitor viatorem interrogat could be rendered as
  The gatekeeper questions the traveler
  or more naturally,
  The gatekeeper asks the traveler

Both are reasonable, depending on how formal or forceful you want the English to sound.

Because once the first part says Ianitor viatorem interrogat, the direct speech naturally follows as what the gatekeeper says to that traveler:

• Quid in sacculo portas?

Latin does not need to repeat to the traveler inside the question. It is already understood from the narrative frame.

This is similar to English:

• The gatekeeper asked the traveler: What are you carrying in the bag?

You would not normally repeat to the traveler again.

A common classroom pronunciation would be roughly:

• Ianitor = yah-NI-tor
• viatorem = wee-ah-TO-rem
• interrogat = in-ter-RO-gat
• quid = kwid
• in sacculo = in SAK-ku-lo
• portas = POR-tas

A fuller pronunciation of the whole sentence:

Yah-ni-tor wee-ah-to-rem in-ter-ro-gat: kwid in sak-ku-lo por-tas?

Exact pronunciation can vary depending on whether you are using restored Classical pronunciation or a more traditional school pronunciation.