La lección de matemáticas es interesante.

Because lección is a feminine noun in Spanish, so it takes the feminine singular article la.

A good rule is:

• el = masculine singular
• la = feminine singular

Here:

• la lección = the lesson

The ending -ción is very often feminine in Spanish:

• la nación
• la canción
• la información
• la lección

So once you learn that pattern, it becomes much easier.

Spanish often uses noun + de + noun where English uses two nouns together.

So:

• la lección de matemáticas = literally the lesson of mathematics
• natural English: the math lesson or the mathematics lesson

This structure is extremely common in Spanish:

• libro de historia = history book
• clase de español = Spanish class
• profesor de inglés = English teacher

So de is doing a lot of work that English often handles just by putting two nouns together.

In Spanish, the school subject is very commonly called las matemáticas, which is plural in form.

So you will often see:

• las matemáticas = math / mathematics

That is just the normal Spanish word for the subject. English can say math as if it were singular, but Spanish usually uses the plural form.

Depending on region and context, you may also hear matemática in the singular, especially in some varieties of Spanish, but matemáticas is extremely standard and very common.

Because ser is usually used for descriptions that are seen as more inherent, general, or identifying.

Here, interesante describes the lesson as being interesting, so es is the standard choice:

• La lección de matemáticas es interesante.

Very broadly:

• ser = what something is
• estar = how something is at the moment / condition / state

That said, in real conversation, está interesante can also appear in some contexts to mean something like it’s getting interesting or it seems interesting right now, but for a basic descriptive sentence like this one, es interesante is the normal choice.

Because not all adjectives change form for gender.

Interesante is an adjective that has the same form for masculine and feminine nouns.

So you get:

• el libro interesante
• la lección interesante

It does change for number, though:

• singular: interesante
• plural: interesantes

Examples:

• La lección es interesante.
• Las lecciones son interesantes.

Because in Spanish, adjectives are often placed after the noun.

So:

• la lección interesante = the interesting lesson

In this sentence, though, the adjective is not directly attached to the noun. It comes after es:

• La lección de matemáticas es interesante.

That structure is:

• subject + verb + adjective

This is very common:

• El libro es bueno.
• La clase es difícil.
• La película es divertida.

So the adjective appears after the verb because it is part of the predicate, not because Spanish is doing anything unusual here.

Normally, yes, you should include la here.

Spanish usually requires an article with a singular countable noun used as the subject of a sentence:

• La lección de matemáticas es interesante.

Without the article:

• Lección de matemáticas es interesante

that sounds incomplete or unnatural in normal speech. It might only work in special contexts like a headline, notes, or a label.

So for a normal full sentence, keep la.

Because after de, when you are naming a subject or category, Spanish often does not use the article.

So:

• clase de español
• libro de historia
• profesor de matemáticas
• lección de matemáticas

Using de las matemáticas would sound more like of the mathematics and usually changes the meaning or sounds too specific for this sentence.

So here, de matemáticas is the natural way to say math lesson.

Yes, you could say that, but the meaning changes slightly.

• lección = lesson
• clase = class

So:

• La lección de matemáticas es interesante. = The math lesson is interesting.
• La clase de matemáticas es interesante. = The math class is interesting.

A lección is more about the content or lesson itself.
A clase can refer to the class session, the course, or the class period.

Both are correct, but they are not exactly the same thing.

Because Spanish uses written accent marks to show which syllable is stressed when it does not follow the default stress rules.

• lección has stress on the last syllable: le-cción
• matemáticas has stress on má: ma-te-má-ti-cas

These accents are part of the correct spelling and should always be written.

They are not optional decoration. They can also sometimes distinguish words:

• si = if
• sí = yes

So it is important to learn them as part of the word.

In most of Latin America, ll is pronounced like y in yes, though the exact sound varies by region.

So lección is roughly:

• lehk-SYON or lek-SYON

A few pronunciation notes:

• ll in this word is not the main issue, because here the spelling is actually lección, with just one l
• the cc before ió gives an s sound in Latin America
• the stressed final part is -ción

So the whole word is roughly:

• lehk-SYON

Not in the same way English does.

English commonly shortens mathematics to math. Spanish normally does not shorten matemáticas to a single everyday standard word in the same way.

So the usual form is simply:

• matemáticas

In informal speech, people may use context to shorten things in other ways, but for a learner, the safe and natural choice is always matemáticas.

The structure is:

article + noun + de + noun + verb + adjective

More specifically:

• La = the
• lección = lesson
• de matemáticas = of math / math
• es = is
• interesante = interesting

So the grammar pattern is:

subject + ser + adjective

This is a very useful beginner pattern:

• El libro es interesante.
• La película es buena.
• La clase es difícil.

Once you know this pattern, you can build many similar sentences very easily.