Learn Hardcore Greek: Λέω στον εαυτό μου να είμαι γενναίος όταν μιλάω ελληνικά μπροστά στην τάξη. - I tell myself to be brave when I speak Greek in front of the class.

Λέω στον εαυτό μου να ηρεμήσω. = I tell myself to calm down.
Λέω στον εαυτό μου ότι όλα θα πάνε καλά. = I tell myself that everything will be fine.

Σκέφτομαι means “I think”, more generally about thoughts, not necessarily “speaking to yourself”:

Σκέφτομαι να πάω διακοπές. = I’m thinking of going on vacation.

In this sentence, the idea is “I tell myself / I encourage myself,” so Λέω στον εαυτό μου is more expressive than just Σκέφτομαι.

Why is it στον εαυτό μου and not something like σε εαυτό μου or τον εαυτό μου?

Στον εαυτό μου breaks down like this:

σε (to) + τον (the, masculine accusative) → στον
εαυτό = “self”
μου = “my”

So στον εαυτό μου = “to myself” (literally: to-the self my).

You need:

σε because of the verb λέω σε κάποιον = “I say to someone”.
The article τον because εαυτός is a noun and normally takes an article.
μου to show whose self it is (myself, yourself, etc.).

Other persons:

Λέω στον εαυτό σου = I say to yourself
Λέει στον εαυτό της = She says to herself
Λέμε στον εαυτό μας = We say to ourselves

Τον εαυτό μου without σε would be used after different verbs, e.g.:

Αγαπώ τον εαυτό μου. = I love myself.
(No σε here, so just τον εαυτό μου.)

Why do we need να είμαι? Why not just Λέω στον εαυτό μου είμαι γενναίος?

In Greek, many verbs that express wishes, intentions, orders, etc., are followed by να and the subjunctive form of the verb.

Λέω στον εαυτό μου να είμαι γενναίος literally =
“I say to myself to be brave.”

Here να είμαι is the subjunctive of είμαι (“to be”), after λέω στον εαυτό μου.
Without να, είμαι would just be a normal present tense statement “I am”:

Είμαι γενναίος. = I am brave.
Λέω στον εαυτό μου να είμαι γενναίος. = I tell myself to be brave.

So να is necessary in this structure; Λέω στον εαυτό μου είμαι γενναίος is not natural Greek.

Why is it γενναίος and not γενναίο or γενναία?

Γενναίος is an adjective meaning “brave”. Adjectives in Greek agree in:

Gender (masculine, feminine, neuter)
Number (singular/plural)
Case (nominative, accusative, etc.)

The implied subject of να είμαι is εγώ (I).
So the adjective must agree with me.

If the speaker is a man/boy → γενναίος (masculine)
If the speaker is a woman/girl → γενναία (feminine)

So a woman would naturally say:

Λέω στον εαυτό μου να είμαι γενναία όταν μιλάω ελληνικά…

Γενναίο is the neuter form; it would agree with a neuter noun, not with εγώ.

What is the difference between μιλάω and μιλώ? Which is better here?

Both μιλάω and μιλώ mean “I speak / I talk”.

μιλάω is the more colloquial / everyday form.
μιλώ is slightly more formal or written, but still common in speech.

In this sentence, μιλάω sounds very natural and conversational:

όταν μιλάω ελληνικά = when I speak Greek

You could also say:

όταν μιλώ ελληνικά

Both are correct; the meaning is the same.
As a learner, you can comfortably use μιλάω in most spoken contexts.

Why is it ελληνικά and not ελληνική or ελληνικό?

Ελληνικά here is the neuter plural form of the adjective ελληνικός (“Greek”).

For languages, Greek usually uses the neuter plural as a kind of adverb or object:

Μιλάω ελληνικά. = I speak Greek.
Μιλάω αγγλικά. = I speak English.
Μιλάω γαλλικά. = I speak French.

So you don’t normally say μιλάω ελληνική.
You use ελληνική (feminine singular) when it modifies a feminine noun:

η ελληνική γλώσσα = the Greek language
η ελληνική μουσική = Greek music

Here there is no noun after ελληνικά, so the “language” meaning is understood, and the standard form is ελληνικά.

Why is it όταν μιλάω ελληνικά and not όταν μιλήσω ελληνικά?

Both are grammatically correct, but they express different aspect (ongoing vs one-time):

όταν μιλάω ελληνικά
- Uses present (imperfective aspect).
- Suggests a repeated / habitual / ongoing action:
  “when(ever) I speak Greek (in general, whenever that happens).”
όταν μιλήσω ελληνικά
- Uses aorist subjunctive (perfective aspect).
- Suggests a single, specific event in the future:
  “when I (finally) speak Greek (that one time).”

In your sentence, the idea is a repeated situation (“whenever I speak Greek in front of the class”), so όταν μιλάω ελληνικά is the natural choice.

Why do we say μπροστά στην τάξη and not just μπροστά τάξη or μπροστά από την τάξη?

Μπροστά usually combines with σε to mean “in front of”:

μπροστά σε κάτι/κάποιον = in front of something/someone

Here:

σε
- την τάξη → στην τάξη
So μπροστά στην τάξη = in front of the class

You cannot drop στην; μπροστά τάξη is incorrect.

What about μπροστά από την τάξη?

μπροστά από also means “in front of” and is common for physical location:
- Μπροστά από το σπίτι. = In front of the house.

But “in front of the class” in the sense of facing the class as an audience is more naturally μπροστά στην τάξη.
You might hear μπροστά από την τάξη for a more literal spatial meaning (e.g. standing in front of the classroom building), but for speaking to the class, μπροστά στην τάξη is the usual phrase.

Could the word order change? For example, can I say Λέω στον εαυτό μου όταν μιλάω ελληνικά να είμαι γενναίος?

Yes, Greek word order is quite flexible, and several variations are possible.
Your example:

Λέω στον εαυτό μου όταν μιλάω ελληνικά να είμαι γενναίος.

is grammatically fine and understandable. The meaning remains the same.

Some common variants:

Λέω στον εαυτό μου να είμαι γενναίος όταν μιλάω ελληνικά μπροστά στην τάξη. (original)
Όταν μιλάω ελληνικά μπροστά στην τάξη, λέω στον εαυτό μου να είμαι γενναίος.
Λέω στον εαυτό μου, όταν μιλάω ελληνικά μπροστά στην τάξη, να είμαι γενναίος.

The differences are mainly in emphasis and style, not in basic meaning.
The original order is very natural and clear, especially for learners.

Can I replace Λέω στον εαυτό μου with Λέω μέσα μου? Do they mean the same thing?

Λέω μέσα μου literally means “I say inside (myself)”, that is, “I say to myself, silently.”

Λέω μέσα μου να είμαι γενναίος… = I say to myself (internally) to be brave…

Λέω στον εαυτό μου and Λέω μέσα μου are very close in meaning, and in many contexts they can be used interchangeably. Slight nuance:

Λέω στον εαυτό μου emphasizes talking to your self as if giving yourself advice or orders.
Λέω μέσα μου emphasizes that the speech is internal (not out loud).

In this sentence, both are natural:

Λέω στον εαυτό μου να είμαι γενναίος…
Λέω μέσα μου να είμαι γενναίος…

How would I say “I told myself to be brave…” instead of “I tell myself…”?

You would change λέω (present) to είπα (aorist past):

Είπα στον εαυτό μου να είμαι γενναίος όταν μιλάω ελληνικά μπροστά στην τάξη.
= I told myself to be brave when I speak Greek in front of the class.

If the speaker is female, you’d still change only the adjective:

Είπα στον εαυτό μου να είμαι γενναία όταν μιλάω ελληνικά μπροστά στην τάξη.

Is εγώ missing here? Why don’t we say Εγώ λέω στον εαυτό μου…?

In Greek, the subject pronoun (like εγώ, “I”) is usually dropped, because the verb ending shows the person:

λέω can only mean “I say” in the present tense.

So:

Λέω στον εαυτό μου… already means “I say to myself…”
Εγώ λέω στον εαυτό μου… is still correct but adds emphasis on “I” specifically, e.g. to contrast with someone else:
- Εγώ λέω στον εαυτό μου να είμαι γενναίος, αλλά αυτός φοβάται.
  = I tell myself to be brave, but he is afraid.

In neutral statements, omitting εγώ is the norm.