Κοίταξα τον εαυτό μου στον καθρέφτη και είδα ότι είχα λίγη οδοντόκρεμα στο πουκάμισο.

Κοίταξα is the aorist of κοιτάζω (to look).

Here it means I looked in the sense of a single completed action:

• Κοίταξα = I looked
• not I was looking or I used to look

So the sentence begins with a quick, finished action: I looked at myself in the mirror...

Greek usually expresses myself with the phrase:

• ο εαυτός μου = myself / literally my self
• τον εαυτό μου = myself in the accusative, because it is the object of the verb

So:

• Κοίταξα τον εαυτό μου = I looked at myself

This is the normal Greek way to say it. Greek does not usually use a single word equivalent to English myself in this kind of sentence.

Also, even a woman would normally say τον εαυτό μου, because εαυτός is a masculine noun.

Because εαυτός behaves like a normal noun in Greek, and Greek normally uses the definite article with it.

So the structure is:

• τον = the (masculine accusative singular)
• εαυτό = self
• μου = my

Literally, it is something like the self of mine, but naturally it means myself.

στον is a contraction of:

• σε + τον → στον

So:

• σε = a very common preposition meaning to / in / at / on
• τον = the (masculine accusative singular)

And:

• καθρέφτη = mirror in the accusative singular

So στον καθρέφτη literally means in/to/at the mirror, but in this sentence the natural English translation is in the mirror.

Because καθρέφτης is the dictionary form (nominative singular), but here the word comes after σε / στον, so Greek uses the accusative:

• ο καθρέφτης = the mirror (nominative)
• τον καθρέφτη = the mirror (accusative)

This is a very common pattern with masculine nouns ending in -ης:

• ο μαθητής → τον μαθητή
• ο καθρέφτης → τον καθρέφτη

ότι means that and introduces a whole clause:

• είδα ότι... = I saw that...

So:

• είδα ότι είχα λίγη οδοντόκρεμα στο πουκάμισο
  = I saw that I had some toothpaste on my shirt

A useful detail: ότι (that) is different from ό,τι (whatever / anything that).

Because the sentence is being told from a past viewpoint.

• είδα = I saw / I noticed
• είχα = I had

Here είχα is the imperfect of έχω, and it describes a state that was true at that moment in the past.

So:

• είδα ότι είχα λίγη οδοντόκρεμα... = I saw that I had some toothpaste...

This is very natural in narrative Greek. It matches the English pattern I saw that I had...

You may also hear present tense after verbs like είδα in casual speech in some contexts, but the version here is the straightforward past narrative choice.

Because οδοντόκρεμα (toothpaste) is:

• feminine
• usually uncountable

So Greek uses λίγη (a little / some) to agree with it:

• λίγη = feminine singular
• οδοντόκρεμα = feminine singular

So:

• λίγη οδοντόκρεμα = a little toothpaste / some toothpaste

Compare:

• λίγος καφές = a little coffee (masculine)
• λίγη οδοντόκρεμα = a little toothpaste (feminine)
• λίγο νερό = a little water (neuter)

Because Greek often leaves out the possessive when it is already obvious from the context.

Since the speaker is talking about looking at themself and noticing toothpaste on the shirt they are wearing, my shirt is understood.

So:

• στο πουκάμισο = on the shirt
• but in context it means on my shirt

You could also say:

• στο πουκάμισό μου

That would be more explicit, and it is also correct.

Because Greek σε is a very broad preposition. It can correspond to English:

• in
• at
• on
• to

So the exact English translation depends on the noun and the context.

Here:

• στο πουκάμισο literally uses σε + το
• but natural English is on the shirt

Likewise:

• στο σπίτι = at home / in the house
• στο τραπέζι = on the table
• στο σχολείο = at school
• στον γιατρό = to the doctor / at the doctor’s

Greek word order is fairly flexible, although the original sentence sounds natural and neutral.

Original:

• Κοίταξα τον εαυτό μου στον καθρέφτη και είδα ότι είχα λίγη οδοντόκρεμα στο πουκάμισο.

You could move things around for emphasis, for example:

• Κοίταξα στον καθρέφτη τον εαυτό μου...
• και είδα ότι στο πουκάμισο είχα λίγη οδοντόκρεμα

But these versions change the focus slightly. The original order is the most straightforward and natural way to say it.