El motivo de la reunión es bastante sencillo.

El is the definite article for a masculine singular noun, like the in English.

Here it goes with motivo, which is a masculine singular noun:

• el motivo = the reason / the motive

Even though reunión is feminine, that does not affect el, because el belongs to motivo, not to reunión.

Because the two nouns have different grammatical genders:

• motivo is masculine → el motivo
• reunión is feminine → la reunión

In Spanish, every noun has a grammatical gender, and the article must match it.

A useful clue here is that many nouns ending in -o are masculine, and many ending in -ión are feminine:

• el motivo
• la reunión
• la decisión
• la explicación

This is a pattern, not an absolute rule, but it helps.

De la reunión means of the meeting and tells you which motivo we are talking about.

So:

• el motivo = the reason
• el motivo de la reunión = the reason for the meeting / the purpose of the meeting

Literally, Spanish often uses de where English might prefer for:

• el libro de María = María’s book / the book of María
• la causa del problema = the cause of the problem
• el motivo de la reunión = the reason for the meeting

So de is very commonly used to connect nouns.

Because in Spanish, nouns often keep their article where English might drop it.

Here, la reunión is a specific noun phrase, so it stays:

• de la reunión = of the meeting

Spanish is generally more comfortable than English with using articles in these structures.

Compare:

• el final de la película
• la puerta de la casa
• el motivo de la reunión

Leaving out la here would sound incomplete or unnatural.

Spanish has two main verbs meaning to be: ser and estar.

Here we use es (from ser) because the sentence is describing an essential characteristic or general nature of the reason:

• El motivo... es sencillo = The reason is simple

You would not normally use estar here, because sencillo is describing what the reason is like, not a temporary state or location.

A very broad guideline:

• ser = identity, characteristics, definitions
• estar = states, conditions, location

So:

• La reunión es importante = The meeting is important
• La reunión está cancelada = The meeting is cancelled

Because the adjective must agree with the noun it describes.

The adjective sencillo describes motivo, not reunión.

• motivo is masculine singular
• so the adjective must also be masculine singular:
  • sencillo

If the noun were feminine, it would change:

• la razón es sencilla

This is called adjective agreement.

Here bastante means something like:

• quite
• fairly
• pretty

In this sentence, it works as an adverb, modifying sencillo:

• bastante sencillo = quite simple / fairly simple

It softens the statement a little compared with something stronger like:

• muy sencillo = very simple

So:

• es sencillo = it is simple
• es bastante sencillo = it is quite simple
• es muy sencillo = it is very simple

It depends on how it is used.

In this sentence, bastante is an adverb, so it does not change:

• es bastante sencillo
• es bastante sencilla
• son bastante sencillos

But bastante can also be an adjective or determiner meaning enough or quite a lot of, and then it can change in number:

• bastante dinero = enough / quite a lot of money
• bastantes problemas = quite a lot of problems

So in your sentence, it stays the same because it modifies an adjective.

Because this is a normal copular sentence with ser:

• El motivo de la reunión = subject
• es = verb
• bastante sencillo = description of the subject

Spanish often describes something using this pattern:

• La casa es grande
• El examen es difícil
• El motivo de la reunión es bastante sencillo

This is different from putting the adjective directly next to the noun, as in:

• un motivo sencillo = a simple reason

Both are possible, but they have different sentence structures.

Yes, you probably could in many contexts, but the nuance may be slightly different.

• motivo = motive, reason, grounds
• razón = reason

In everyday use, both can often overlap. But motivo can sometimes sound a little more like the cause, purpose, or underlying reason.

So:

• El motivo de la reunión...
• La razón de la reunión...

Both are possible, though motivo may sound a bit more natural in some formal or explanatory contexts.

The standard and most natural order is:

• El motivo de la reunión es bastante sencillo.

Spanish does allow some flexibility, but changing the order usually changes the emphasis.

For example:

• Bastante sencillo es el motivo de la reunión — grammatically possible, but unusual and marked
• El motivo es bastante sencillo — also fine if the context already makes clear de la reunión

So for normal use, the original order is the best choice.

Reunión has a written accent because of Spanish stress rules.

It is stressed on the last syllable:

• reu-nión

Words ending in -n, -s, or a vowel are normally stressed on the second-to-last syllable unless there is an accent mark. Since reunión is stressed on the last syllable and ends in -n, it needs the accent.

The accent mark also helps show the correct pronunciation clearly.

In Spain Spanish, reunión is pronounced roughly like:

• reh-oo-NYON

A few helpful points:

• r at the start is a tapped or light rolled Spanish r
• eu is pronounced as two vowels in sequence, not like English you
• the stress falls on -ón
• ión sounds roughly like yon, with a Spanish y-like sound before the o

So the stress is very important:

• reu-NIÓN

Yes. Depending on context, sencillo can suggest:

• simple
• straightforward
• uncomplicated
• plain
• easy to understand

In this sentence, it likely means the reason is not complicated or easy to explain.

So although you may have been shown one English meaning, learners should know that sencillo can cover a small range of related ideas depending on context.