El desmontaje empieza después del montaje.

Because both words are masculine singular nouns in Spanish:

• el desmontaje = the dismantling / disassembly
• el montaje = the assembly / setup

In this sentence, they are being used as things or events, not as verbs, so Spanish normally uses the definite article el.

A very literal breakdown is:

• El desmontaje = the dismantling
• empieza = begins
• después del montaje = after the assembly

Spanish often uses the article where English may or may not use the.

They are opposite ideas:

• montaje = assembly, putting something together, setup
• desmontaje = disassembly, dismantling, taking something apart

The prefix des- often adds the idea of undoing, reversing, or removing the action of the base word.

So:

• montar = to assemble / mount / put together
• desmontar = to dismantle / take apart

And the nouns formed from them are:

• montaje
• desmontaje

Yes. That is exactly the pattern.

• montaje comes from montar
• desmontaje comes from desmontar

The prefix des- is very common in Spanish and often works a bit like English un-, dis-, or de-, depending on the word.

Examples:

• hacer = to do, make
  deshacer = to undo
• orden = order
  desorden = disorder
• conectar = to connect
  desconectar = to disconnect

So desmontaje is the noun for the reverse of montaje.

Because de + el contracts to del in Spanish.

So:

• después de el montaje → después del montaje

This contraction is mandatory with el.

A similar contraction happens with a + el = al.

Examples:

• Voy al centro. = I’m going to the centre.
• Después del montaje... = After the assembly...

Important: this contraction does not happen with la.

• después de la reunión = after the meeting

Because the sentence needs a conjugated verb, not the infinitive.

The subject is El desmontaje, which is third person singular, so the verb must match it:

• El desmontaje empieza... = The dismantling begins...

The infinitive empezar means to begin, but it cannot work by itself here as the main verb.

Present tense of empezar:

• yo empiezo
• tú empiezas
• él/ella/usted empieza
• nosotros empezamos
• vosotros empezáis
• ellos/ellas/ustedes empiezan

Because empezar is an e → ie stem-changing verb in most present-tense forms.

So the e in the stem changes to ie when stressed:

• empiezo
• empiezas
• empieza
• empiezan

But not in nosotros or vosotros:

• empezamos
• empezáis

This is a very common pattern in Spanish. Other verbs like this include:

• pensar → piensa
• cerrar → cierra
• comenzar → comienza

Yes. El desmontaje comienza después del montaje is also correct.

Both empezar and comenzar mean to begin/start. In many contexts they are interchangeable.

Very roughly:

• empezar often feels a little more everyday and common
• comenzar can feel slightly more formal or polished

But both are normal and correct in Spain.

Because desmontaje is a masculine noun.

Many nouns ending in -aje are masculine:

• el viaje = the trip
• el garaje = the garage
• el paisaje = the landscape
• el montaje = the assembly
• el desmontaje = the dismantling

So the correct article is el, not la.

Yes. That is also correct.

Spanish word order is more flexible than English, as long as the meaning stays clear.

These are both fine:

• El desmontaje empieza después del montaje.
• Después del montaje empieza el desmontaje.

The second version puts more emphasis on the time sequence: After the assembly, the dismantling begins.

Because the subject is already stated clearly: El desmontaje.

Spanish usually does not add an extra subject pronoun when it is unnecessary. In fact, Spanish often avoids pronouns unless they are needed for emphasis or contrast.

So:

• El desmontaje empieza... is natural.
• Adding something like él here would usually sound unnecessary, and ello would not be used this way.

Use desmontaje when you need a noun: the process, event, or act of dismantling.

Use desmontar when you need a verb: to dismantle.

Compare:

• El desmontaje empieza después del montaje.
  = The dismantling begins after the assembly.

• Van a desmontar la estructura mañana.
  = They are going to dismantle the structure tomorrow.

A useful way to think about it:

• montar / desmontar = actions
• montaje / desmontaje = the action as a thing or process

Después means after or afterwards.

In this sentence it appears in the structure:

• después de + noun
• después del montaje = after the assembly

You can also use it without a following noun:

• Primero montan todo. Después, lo desmontan.
  = First they assemble everything. Afterwards, they dismantle it.

So here:

• después del montaje tells you when the dismantling begins.

A broad Spain pronunciation would be approximately:

• El = ehl
• desmontaje = des-mon-TA-he
• empieza = em-PYE-tha or em-PYE-sa, depending on accent
• después = des-PWES
• del = del
• montaje = mon-TA-he

A couple of useful notes for Spain:

• In much of Spain, z and soft c are pronounced like th in think
• The j sound in montaje is a strong throaty sound, like the ch in Scottish loch

So in much of Spain, empieza sounds like em-PYE-tha.