La primera escena del documental me emocionó tanto que casi olvidé las palomitas.

Because de + el contracts to del in Spanish.

• de el documental → del documental
• This contraction is required in standard Spanish.

The only common exception is when el is part of a proper name, for example de El Escorial.

Because the subject of the verb is La primera escena del documental, not I.

So the structure is:

• La primera escena del documental = the subject
• me = me
• emocionó = moved / affected emotionally

Literally, it works like this:

• The first scene of the documentary moved me so much...

If you said emocioné, it would mean I moved (someone emotionally), which is not the intended meaning here.

Me is the object pronoun meaning me.

In this sentence, the scene is doing something to the speaker:

• La escena me emocionó = The scene moved me

Spanish often uses object pronouns before a conjugated verb:

• me
• te
• lo / la
• nos
• os
• los / las

So me emocionó means it moved me.

Because with a normal conjugated verb, unstressed object pronouns usually go before the verb in Spanish.

So:

• me emocionó
• te ayudó
• nos sorprendió

They can attach to the end only with:

• infinitives: emocionarme
• gerunds: emocionándome
• affirmative commands: emocióname

But here the verb is a normal finite form, so me goes before emocionó.

Tanto que means so much that.

It shows a consequence:

• Me emocionó tanto que casi olvidé las palomitas.
• It moved me so much that I almost forgot the popcorn.

This is a very common structure in Spanish:

• llovió tanto que... = it rained so much that...
• gritó tanto que... = he/she shouted so much that...

Here, tanto modifies the verb emocionó.

Because the sentence describes specific completed events in a narrative.

• emocionó = it moved me
• olvidé = I forgot

The preterite is natural here because the speaker is talking about a particular moment:

• the first scene affected me
• I almost forgot the popcorn

If you used the imperfect, it would suggest background, repetition, or an ongoing state, which does not fit as well here.

Because olvidar can be used directly with an object:

• olvidé las llaves
• olvidé las palomitas

That is the simplest and most neutral structure here.

There is also olvidarse de:

• me olvidé de las llaves
• me olvidé de las palomitas

That also exists and is common, but it is a different construction.

So both are possible in many contexts:

• olvidé las palomitas
• me olvidé de las palomitas

In this sentence, olvidé las palomitas is perfectly natural.

Spanish often uses the definite article where English might use no article.

Here las palomitas refers to the popcorn the speaker had with them, so the article sounds natural.

Compare:

• Compré palomitas. = I bought popcorn.
• Casi olvidé las palomitas. = I almost forgot the popcorn.

In the second sentence, it sounds like a specific, identifiable thing in the situation.

Because in Spanish, palomitas is normally used in the plural to mean popcorn.

This is standard and very natural in Spain:

• comer palomitas
• unas palomitas
• las palomitas

Even though English treats popcorn as a mass noun, Spanish commonly treats it as a plural noun made up of many little pieces.

Because ordinal numbers like primero / primera usually come before the noun in ordinary Spanish:

• la primera escena
• el primer día
• la segunda parte

Escena primera can exist in certain formal, literary, or theatrical contexts, but for normal everyday Spanish, la primera escena is the standard choice.

Because escena is a feminine singular noun.

So the ordinal has to agree with it:

• primer / primero for masculine singular
• primera for feminine singular

Examples:

• el primer documental
• la primera escena

Agreement in gender and number is very important in Spanish.

Yes. Spanish word order is more flexible than English.

You could say:

• La primera escena del documental me emocionó tanto que...
• Me emocionó tanto la primera escena del documental que...

Both are grammatical.

The difference is mainly emphasis:

• starting with La primera escena... focuses first on the scene
• starting with Me emocionó tanto... focuses first on the emotional effect

The original version is very natural and straightforward.

The accent marks show the correct stress and also help identify the verb forms.

• emocionó = third person singular preterite
  (he/she/it moved emotionally)
• olvidé = first person singular preterite
  (I forgot)

Without the written accents, the forms would either be incorrect or could be confused with other forms.

These accents are very common in preterite endings:

• hablé
• comí
• vivió
• llegó

Not here.

Use tanto with verbs:

• me emocionó tanto
• trabajó tanto
• llovió tanto

Use tan before adjectives or adverbs:

• tan emocionante = so moving
• tan rápido = so quickly

So in this sentence, because emocionó is a verb, tanto is the correct choice.