Άφησα την οδοντόβουρτσά μου δίπλα στον νιπτήρα.

Because Greek verbs usually show the subject by their ending. Άφησα is the 1st person singular form, so it already means I left.

You can add εγώ if you want emphasis or contrast:

• Εγώ άφησα την οδοντόβουρτσά μου... = I left my toothbrush...

But in a neutral sentence, Greek normally leaves the subject pronoun out.

Άφησα is the aorist of αφήνω. In sentences like this, it usually corresponds to a simple past in English:

• αφήνω = I leave / am leaving
• άφησα = I left

So Άφησα την οδοντόβουρτσά μου... means a completed past action: I left my toothbrush...

Not necessarily. Άφησα can mean I left in the sense of I put / placed / left behind. The context decides whether it sounds intentional or accidental.

In this sentence, it most naturally means:

• I left / put my toothbrush next to the sink

If you wanted to clearly say you forgot it there, Greek would usually make that more explicit from context or wording.

This is very normal in Greek. Greek usually uses the definite article together with a possessive clitic.

So:

• την οδοντόβουρτσά μου = literally the toothbrush my
• natural English = my toothbrush

This pattern is extremely common:

• το βιβλίο μου = my book
• η αδερφή μου = my sister
• τα κλειδιά μου = my keys

So even though it feels unusual from an English point of view, it is standard Greek.

Because μου here is an enclitic possessive form, and in normal Greek it usually comes after the noun:

• η οδοντόβουρτσά μου = my toothbrush

If you want stronger emphasis, Greek can use a fuller structure such as:

• η δική μου οδοντόβουρτσα = my toothbrush / the toothbrush that is mine

But the ordinary, everyday pattern is noun + μου.

Because this phrase is the direct object of the verb Άφησα.

The dictionary form is:

• η οδοντόβουρτσα = the toothbrush (nominative)

But after the verb, as the thing being left, it goes into the accusative:

• την οδοντόβουρτσα = the toothbrush

So:

• Άφησα την οδοντόβουρτσά μου = I left my toothbrush

This happens because μου is an enclitic. When a word stressed on the third-from-last syllable is followed by an enclitic, Greek often adds an extra written accent to show the stress pattern more clearly.

So:

• οδοντόβουρτσα
• η οδοντόβουρτσά μου

This does not mean it is a different word or a different case. It is mainly an accentuation rule triggered by μου.

You will see the same kind of thing in other phrases too.

Δίπλα means next to, beside, or by.

It is often used with σε + a noun:

• δίπλα σε κάτι = next to something

In your sentence, that becomes:

• δίπλα στον νιπτήρα = next to the sink

So δίπλα gives the location of where the toothbrush was left.

Because σε + τον normally contracts to στον in modern Greek.

So:

• σε τον νιπτήρα → στον νιπτήρα

This kind of contraction is very common:

• σε το → στο
• σε τη / στην → στη / στην
• σε τον → στον

So δίπλα στον νιπτήρα is the normal form.

Because after σε (here inside στον) the noun is in the accusative.

The basic form is:

• ο νιπτήρας = the sink (nominative)

But after σε, it becomes:

• τον νιπτήρα = the sink (accusative)

So:

• δίπλα στον νιπτήρα = next to the sink

This is a very common masculine pattern:

• ο γιατρός → τον γιατρό
• ο δρόμος → τον δρόμο
• ο νιπτήρας → τον νιπτήρα

No, Greek word order is more flexible than English word order. The sentence

• Άφησα την οδοντόβουρτσά μου δίπλα στον νιπτήρα.

is a neutral, natural way to say it.

But Greek can change the order for emphasis, for example:

• Την οδοντόβουρτσά μου άφησα δίπλα στον νιπτήρα.
• Δίπλα στον νιπτήρα άφησα την οδοντόβουρτσά μου.

These all mean roughly the same thing, but the focus changes slightly depending on what you want to highlight.

A rough guide is:

• AH-fi-sa tin o-don-TO-vour-TSA mou THEE-pla ston ni-pti-RA

A few helpful notes:

• θ sounds like th in this
• δ sounds like th in this as well in modern Greek pronunciation
• μπ, ντ, γκ can have special values, but none of those appear here
• the stressed syllables are important:
  • Άφησα
  • οδοντόβουρτσά
  • δίπλα
  • νιπτήρα

Listening to native audio is especially useful with words like οδοντόβουρτσα, because the written form is much easier than producing it smoothly at speed.