Learn Hardcore Greek: Χτες υποσχέθηκες να μου στείλεις το βίντεο, αλλά το ξέχασες. - Yesterday you promised to send me the video, but you forgot.

Χτες, χθες, and εχτές are all common spellings/pronunciations of the same word.
χθες is more formal / older style.
χτες and εχτές are more colloquial and very common in speech.

In everyday modern Greek, you can safely use χτες or εχτές; they are understood the same way.

What is the verb υποσχέθηκες, and what tense/person is it?

υποσχέθηκες means you promised.

Grammatically:

Verb: υπόσχομαι = to promise
υποσχέθηκες is:
- aorist tense (simple past)
- 2nd person singular
- middle voice form (because υπόσχομαι is a deponent-type verb)

So Χτες υποσχέθηκες… = Yesterday you promised…

Why is there no word for to as in “to send”, and instead we have να μου στείλεις?

Modern Greek does not use an infinitive like English to send.
Instead, it uses να + a verb in the subjunctive:

να στείλεις ≈ to send

After verbs like υπόσχομαι (I promise), θέλω (I want), ελπίζω (I hope), etc., Greek usually uses a να-clause:

υποσχέθηκες να στείλεις… = you promised to send…

What does μου mean in να μου στείλεις, and why is it placed there?

μου means to me or my depending on context. Here it is an unstressed object pronoun:

να μου στείλεις = to send me / to send to me

About the position:

Unstressed object pronouns like μου, σου, τον, την, το normally go before the verb in a να-clause:
- να μου στείλεις (correct)
- να στείλεις μου (wrong)

So να μου στείλεις το βίντεο = to send me the video.

What is the difference between στείλεις and στέλνεις?

Both come from στέλνω = to send, but they express different aspects:

στέλνεις: present tense, indicative
- Στέλνεις το βίντεο τώρα. = You are sending the video now.
στείλεις: aorist subjunctive (used with να)
- να στείλεις το βίντεο = to send the video (once, as a complete action)

In this sentence, να μου στείλεις expresses a single, complete action in the future (the act of sending the video at some point), which is why the aorist subjunctive στείλεις is used instead of στέλνεις.

Why does the sentence use το βίντεο with the article το? Could we say just βίντεο?

το is the definite article for neuter singular nouns, like:

το βίντεο = the video

We use the definite article here because a specific video is meant (the one both speakers know about).

You could say να μου στείλεις βίντεο (without το) but the meaning becomes more general or indefinite, like to send me (some) video / send video, not necessarily that one specific file.

So το βίντεο is natural and preferred here because both people clearly know which video they are talking about.

In αλλά το ξέχασες, what does το refer to?

το is a neuter object pronoun meaning it.

Here it refers back to το βίντεο:

το ξέχασες = you forgot it
το = το βίντεο

Greek avoids repeating the full noun if it is clear from the context, so instead of αλλά ξέχασες το βίντεο, it uses αλλά το ξέχασες.

What exactly does ξέχασες mean, and what tense is it?

ξέχασες means you forgot.

Verb: ξεχνάω / ξεχνώ = to forget

ξέχασες: aorist tense, 2nd person singular
- χτες ξέχασες = yesterday you forgot
Present tense would be ξεχνάς = you forget / you are forgetting
Imperfect past would be ξεχνούσες = you kept forgetting / you used to forget

In this sentence, το ξέχασες describes one completed past action, so the aorist is used.

Why is there no εσύ in the Greek sentence? How do we know it means you?

Greek verb endings show the person, so the subject pronoun is usually optional.

υποσχέθηκες already means you (singular) promised
ξέχασες already means you (singular) forgot

You can add εσύ for emphasis or contrast:

Εσύ χτες υποσχέθηκες… = You (yourself) promised yesterday…

But in neutral statements like this, the bare verb form is enough, so the sentence naturally omits εσύ.

Can we change the word order, for example: Χτες μου υποσχέθηκες να στείλεις το βίντεο?

Yes. Greek word order is relatively flexible.

Possible variants include:

Χτες μου υποσχέθηκες να μου στείλεις το βίντεο.
Μου υποσχέθηκες χτες να μου στείλεις το βίντεο.

Notes:

Unstressed pronouns like μου stay very close to the verb:
- μου υποσχέθηκες, μου στείλεις, το ξέχασες
Moving adverbs like χτες (yesterday) is usually fine:
- Χτες υποσχέθηκες…
- Υποσχέθηκες χτες…

The original word order is very natural, but these alternatives are also grammatically correct.

Why is it να μου στείλεις and not να στείλεις με for to send me?

Greek has different unstressed pronouns for to me and me:

μου = to me (indirect object; old dative)
με = me (direct object; accusative)

With στέλνω (to send), the person is usually an indirect object:

να μου στείλεις το βίντεο = to send the video to me

Using με would mean something like to send me (as a parcel), where me is the main object being sent:

να με στείλεις στο γραφείο = to send me to the office

So here μου is the correct choice.

Could we also say Χτες μου υποσχέθηκες ότι θα μου στείλεις το βίντεο instead of να μου στείλεις? What is the difference?

Yes, that is also correct:

Χτες μου υποσχέθηκες ότι θα μου στείλεις το βίντεο.
= Yesterday you promised me that you will send me the video.

Difference in structure:

να + subjunctive:
- μου υποσχέθηκες να μου στείλεις (you promised to send me)
- Shorter, very natural, feels a bit more direct/verb-like.
ότι + future:
- μου υποσχέθηκες ότι θα μου στείλεις (you promised that you will send me)
- Explicitly states the future action as a statement.

In everyday speech, both patterns are common and acceptable; the meaning is practically the same here.

Why is μου used only once? In English we say “you promised to send me the video” but Greek could say μου υποσχέθηκες να μου στείλεις….

Greek can repeat μου, but it doesn’t have to in this shorter form.

Three options:

Υποσχέθηκες να μου στείλεις το βίντεο.
- Only μου with στείλεις
- Means: You promised to send me the video.
- Very natural and idiomatic.
Μου υποσχέθηκες να μου στείλεις το βίντεο.
- Double μου: one with υποσχέθηκες (you promised me) and one with στείλεις (to send me)
- Also correct, just a bit more explicit/redundant.
Μου υποσχέθηκες να στείλεις το βίντεο.
- μου only with υποσχέθηκες (you promised me)
- The to me is understood from context; still natural.

The given sentence (without μου after υποσχέθηκες) is perfectly fine; Greek often avoids repeating pronouns if the meaning is clear.