Dejé el móvil debajo del pupitre y no me di cuenta hasta el final de la clase.

Dejé is the pretérito indefinido (simple past), which is very common in Spain for a completed action in the past.

Here, dejé el móvil debajo del pupitre means the speaker did one completed action: they left the phone there.

In much of Spain, the pretérito perfecto (he dejado) is often used for recent past actions connected to the present, but dejé is still perfectly normal when you are telling a past event as a finished story.

So this sentence presents the situation as a completed sequence:

• Dejé el móvil...
• y no me di cuenta...
• hasta el final...

It sounds like a narrated past event.

In Spanish, it is very common to use the definite article (el, la, los, las) instead of a possessive adjective when ownership is obvious from context.

So:

• Dejé el móvil... = I left my phone...

Because the speaker is obviously talking about their own phone, Spanish does not need mi.

This happens a lot:

• Me duele la cabeza = My head hurts
• Me lavé las manos = I washed my hands
• Cogí el abrigo = I picked up my coat

Using mi móvil would also be possible, but el móvil sounds very natural and normal here.

Because debajo de is the full expression meaning under / underneath.

So the structure is:

• debajo de + noun

Examples:

• debajo de la mesa = under the table
• debajo del pupitre = under the desk

del is just the contraction of:

• de + el = del

So:

• debajo de el pupitre → debajo del pupitre

You cannot say debajo el pupitre here because debajo needs de.

Pupitre usually means a school desk, especially the kind a student uses in class.

In this sentence, it strongly suggests a classroom setting, which fits with al final de la clase.

Depending on context:

• mesa = table
• escritorio = desk, often for work/office/home
• pupitre = school desk

So debajo del pupitre means under the school desk.

Darse cuenta (de) is an idiomatic expression meaning:

• to realize
• to notice
• to become aware

So:

• no me di cuenta = I didn’t realize / I didn’t notice

This expression is not translated word for word. Literally, it comes from dar cuenta, but in real Spanish you should learn darse cuenta de as a fixed phrase.

Structure:

• me = reflexive pronoun
• di = first person singular preterite of dar
• cuenta = part of the idiom

Examples:

• Me di cuenta de mi error. = I realized my mistake.
• No se dio cuenta. = He/She didn’t notice.

In your sentence, the thing realized is understood from context: the speaker didn’t notice that they had left the phone there.

Normally, the full expression is darse cuenta de + something.

For example:

• Me di cuenta de que había perdido el móvil.
• Me di cuenta del error.

But Spanish often leaves out de + something when the thing is obvious from context.

So here:

• no me di cuenta hasta el final de la clase

means something like:

• I didn’t realize it until the end of class
• I didn’t notice (what had happened) until the end of class

The omitted idea is clear: the speaker didn’t realize they had left the phone under the desk.

Yes, dar is irregular in the preterite.

Its forms are:

• di
• diste
• dio
• dimos
• disteis
• dieron

So:

• me di cuenta = I realized / I noticed

Even though dar usually means to give, in darse cuenta it forms an idiomatic expression.

This is a very useful chunk to memorize:

• me di cuenta
• te diste cuenta
• se dio cuenta
• nos dimos cuenta

Me is the reflexive pronoun used with the verb darse in the expression darse cuenta.

The full infinitive is:

• darse cuenta (de)

When conjugated, the pronoun changes:

• me doy cuenta
• te das cuenta
• se da cuenta
• nos damos cuenta

In the past:

• me di cuenta
• te diste cuenta
• se dio cuenta

You should learn it as one unit, not as dar + cuenta separately.

Because hasta adds the meaning of not until.

Compare:

• No me di cuenta hasta el final de la clase.
  = I didn’t realize until the end of class.

• Me di cuenta al final de la clase.
  = I realized it at the end of class.

With no ... hasta, Spanish expresses the idea of something not happening before a certain point.

So in your sentence:

• no me di cuenta hasta... = I didn’t realize until...

In this context, la clase most naturally means the class / the lesson / the period of class.

So:

• el final de la clase = the end of the class / lesson

In English, we often say at the end of class rather than at the end of the lesson, but both ideas are possible depending on context.

Since the sentence mentions pupitre, it clearly sounds like a school or classroom situation.

Not exactly. Dejar means to leave something somewhere.

So:

• Dejé el móvil debajo del pupitre = I left the phone under the desk

In context, English might sometimes translate this more naturally as I forgot my phone under the desk, but the Spanish verb itself is still dejar, not olvidar.

Compare:

• Dejé el móvil en casa. = I left my phone at home.
• Olvidé el móvil en casa. = I forgot my phone at home.

Often both can fit similar situations, but they focus on slightly different things:

• dejar = where you left it
• olvidar = the act of forgetting

No. In standard Spanish, no goes before the verb phrase, and object/reflexive pronouns like me usually go before the conjugated verb.

So the correct order is:

• no me di cuenta

Breakdown:

• no = negation
• me = pronoun
• di = conjugated verb
• cuenta = part of the expression

Spanish word order here is quite fixed:

• No me di cuenta
• No lo vi
• No se acordó

Me no di cuenta is not correct.

Usually no, at least not with exactly the same meaning.

Darse cuenta means to realize / notice something yourself.

Enterarse de usually means to find out / hear about something, often from information or news.

Compare:

• Me di cuenta de que había dejado el móvil debajo del pupitre.
  = I realized / noticed that I had left the phone under the desk.

• Me enteré de que habían encontrado mi móvil.
  = I found out that they had found my phone.

So in your sentence, me di cuenta is the natural choice because the speaker became aware of their own mistake.

Yes, móvil is especially associated with Spain for mobile phone / cell phone.

In many parts of Latin America, people more often say:

• celular
• sometimes teléfono

So in Spain:

• el móvil = the phone / mobile phone

This is one of the clearest vocabulary clues that the sentence is aimed at European Spanish.

Yes, if you want to make possession explicit.

Compare:

• debajo del pupitre = under the desk
• debajo de mi pupitre = under my desk

In many situations, del pupitre is enough because it is understood to be the speaker’s desk. But if you want extra clarity or emphasis, de mi pupitre is perfectly correct.

So both are possible:

• Dejé el móvil debajo del pupitre
• Dejé el móvil debajo de mi pupitre

The version with del pupitre sounds very natural when the context already makes it obvious.