Η ανιψιά μου ρώτησε ποιο λουλούδι να βάλει στο βάζο και διάλεξε το κόκκινο.

In Greek, a specific possessed noun usually keeps the definite article, so η ανιψιά μου is the normal way to say my niece.

Also, the weak possessive pronoun μου normally comes after the noun:

• η ανιψιά μου = my niece
• το βιβλίο μου = my book

If you want extra emphasis, Greek can use δικός:

• η δική μου ανιψιά = my own niece / my niece specifically

ρώτησε is the aorist (simple past), 3rd person singular, from ρωτάω / ρωτώ.

Here it means she asked and presents the action as one completed event.

Because λουλούδι is a neuter singular noun, and ποιο must agree with it.

The forms are:

• ποιος for masculine
• ποια for feminine
• ποιο for neuter

So:

• ποιο λουλούδι = which flower

They are not the same.

• ποιο λουλούδι = which flower, meaning one from a set of possible choices
• τι λουλούδι = what kind of flower, meaning the type or category

In this sentence, she is choosing among flowers, so ποιο is the natural word.

Modern Greek normally does not use an infinitive the way English does. Instead, it usually uses να + subjunctive.

So:

• ποιο λουλούδι να βάλει literally works like which flower that she should put
• but in natural English it becomes which flower to put

This is a very common Greek pattern.

After να, Greek chooses between different aspects.

• να βάλει is aorist subjunctive: one complete action
• να βάζει is present subjunctive: repeated, ongoing, or habitual action

Here the idea is a single act of putting one flower in the vase, so να βάλει is the right choice.

στο is the contracted form of σε το.

So:

• σε + το = στο

In this sentence, στο βάζο means in the vase or into the vase. With the verb βάλει, the sense is usually into the vase.

Because Greek often leaves the subject unstated when it is already clear from context.

The sentence starts with Η ανιψιά μου, and the next verb διάλεξε clearly continues with the same subject. Greek does this very often, much more freely than English.

διάλεξε is the aorist of διαλέγω, which means choose or pick.

So:

• διαλέγω = I choose
• διάλεξε = she chose / she picked

Because Greek can leave out a noun when it has already been mentioned and is easy to understand from context.

Earlier we had λουλούδι, so later το κόκκινο means:

• the red one
• more literally here, the red flower

This is a very common Greek pattern: article + adjective can stand in for the full noun phrase.

Because it refers back to λουλούδι, and λουλούδι is neuter.

Greek adjectives must agree with the noun in:

• gender
• number
• case

So:

• το κόκκινο λουλούδι = the red flower

If the noun were feminine or masculine, the adjective form would change too.

It is the direct object of διάλεξε, so its function is accusative.

However, in the neuter singular, the nominative and accusative forms are the same, so you still see:

• το κόκκινο

So the form does not change, but its role in the sentence is accusative.