Στοιβάζω χαρτοκιβώτια.

Στοιβάζω is the 1st person singular present form of the verb. The ending -ω here tells you the subject is I.

So the verb form itself already includes the subject:

• στοιβάζω = I stack / I am stacking

This is why Greek does not need a separate word for I unless the speaker wants emphasis:

• Εγώ στοιβάζω = I stack (with extra emphasis on I)

Greek often leaves out subject pronouns because the verb ending already shows who the subject is.

So:

• Στοιβάζω χαρτοκιβώτια.

naturally means:

• I stack boxes
• I am stacking boxes

Adding εγώ is possible, but it usually adds emphasis or contrast:

• Εγώ στοιβάζω χαρτοκιβώτια, όχι αυτός.
• I’m stacking boxes, not him.

In Greek, the present tense can cover both ideas, depending on context.

So στοιβάζω can mean:

• I stack
• I am stacking

The Greek present tense is imperfective, so it can describe:

• a repeated action
• a habitual action
• an action happening now

Context tells you which English translation fits best.

Χαρτοκιβώτια is neuter plural and here it is the direct object of the verb.

The singular is:

• χαρτοκιβώτιο = box / carton

The plural is:

• χαρτοκιβώτια = boxes / cartons

This is a common Greek neuter pattern:

• -ιο in the singular
• -ια in the plural

For example:

• βιβλίο → βιβλία
• χωριό → χωριά
• χαρτοκιβώτιο → χαρτοκιβώτια

Good question. In Greek, neuter nouns often have the same form in the nominative and accusative.

So for the plural:

• nominative: χαρτοκιβώτια
• accusative: χαρτοκιβώτια

Here it is the accusative plural, because it is the object of στοιβάζω.

You know it is the object from the sentence structure and the verb, not from a special ending.

Without an article, the noun is usually understood in a more general or indefinite way.

So:

• Στοιβάζω χαρτοκιβώτια. = I stack boxes / I’m stacking boxes.

If you want the boxes, you would normally say:

• Στοιβάζω τα χαρτοκιβώτια.

So the article changes the feel:

• χαρτοκιβώτια = boxes in general / some boxes
• τα χαρτοκιβώτια = the boxes

A rough pronunciation guide is:

• Στοιβάζω → stee-VA-zo
• χαρτοκιβώτια → har-to-kee-VO-tya

A few useful notes:

• οι is pronounced like ee
• χ is not like English h; it is a throat sound, like the ch in Scottish loch or German Bach
• the stress is on:
  • -βά- in στοιβάζω
  • -βώ- in χαρτοκιβώτια

Greek word order is more flexible than English word order.

Στοιβάζω χαρτοκιβώτια is a perfectly natural neutral order here.

But you may also hear:

• Χαρτοκιβώτια στοιβάζω.

That changes the emphasis, putting more focus on χαρτοκιβώτια.

So:

• Στοιβάζω χαρτοκιβώτια = neutral
• Χαρτοκιβώτια στοιβάζω = more marked, with focus on boxes

Yes, it is one word, and it is a compound noun.

It comes from:

• χαρτί = paper
• κιβώτιο = box / crate

So χαρτοκιβώτιο literally has the idea of a paper box, which in normal usage means a cardboard box / carton.

Its plural is:

• χαρτοκιβώτια

Usually yes. Στοιβάζω is normally transitive, meaning you stack something.

So it commonly appears with a direct object, like:

• Στοιβάζω χαρτοκιβώτια.
• I stack boxes.

You can of course make the object more specific:

• Στοιβάζω τα χαρτοκιβώτια στην αποθήκη.
• I’m stacking the boxes in the warehouse.

So in your sentence, χαρτοκιβώτια is the thing being stacked.